Silence new -Wmissing-home-modules warning for cabal-install

[hvr]

This should hopefully silence all 4 combinations of flag(lib)
and flag(monolithic).

cabal-install.cabal would greatly benefit from common stanzas (#2832)
as it would help reduce the redundancy significantly.

[mention-bot]

@hvr, thanks for your PR! By analyzing the history of the files in this pull request, we identified @dcoutts, @ezyang and @23Skidoo to be potential reviewers.

[hvr]

For the record, here's how GHC warned about it:

Preprocessing executable 'cabal' for cabal-install-2.1.0.0..
Building executable 'cabal' for cabal-install-2.1.0.0..

<no location info>: warning: [-Wmissing-home-modules]
    Modules are not listed in command line: Paths_cabal_install
                                            UnitTests.Distribution.Client.ArbitraryInstances
                                            UnitTests.Distribution.Client.FileMonitor
                                            UnitTests.Distribution.Client.GZipUtils
                                            UnitTests.Distribution.Client.Glob
                                            UnitTests.Distribution.Client.IndexUtils.Timestamp
                                            UnitTests.Distribution.Client.InstallPlan
                                            UnitTests.Distribution.Client.JobControl
                                            UnitTests.Distribution.Client.ProjectConfig
                                            UnitTests.Distribution.Client.Sandbox
                                            UnitTests.Distribution.Client.Sandbox.Timestamp
                                            UnitTests.Distribution.Client.Store
                                            UnitTests.Distribution.Client.Tar
                                            UnitTests.Distribution.Client.Targets
                                            UnitTests.Distribution.Client.UserConfig
                                            UnitTests.Distribution.Solver.Modular.Builder
                                            UnitTests.Distribution.Solver.Modular.DSL
                                            UnitTests.Distribution.Solver.Modular.DSL.TestCaseUtils
                                            UnitTests.Distribution.Solver.Modular.MemoryUsage
                                            UnitTests.Distribution.Solver.Modular.QuickCheck
                                            UnitTests.Distribution.Solver.Modular.RetryLog
                                            UnitTests.Distribution.Solver.Modular.Solver
                                            UnitTests.Distribution.Solver.Modular.WeightedPSQ
                                            UnitTests.Options
[ 1 of 29] Compiling IntegrationTests2 ( tests/IntegrationTests2.hs, /stuff3/work/GitHub/cabal/dist-newstyle/build/x86_64-linux/ghc-8.2.0.20170522/cabal-install-2.1.0.0/build/cabal/cabal-tmp/IntegrationTests2.o )

[hvr]

One would hope that Travis is clever enough to simply reuse the build report it already had if you push an already validated commit-hash from a single commit PR... :-/