Add missing Threads dependency to Config.cmake.in

[sum01]

Without this, systems that linked against Threads::Threads (namely
Linux) don't "just work" with find_package(SQLiteCpp), but instead
require you to find & link against Threads manually.

So this fixes that by finiding it for you, if it was used.

[coveralls]

Coverage remained the same at 100.0% when pulling 42f476d on sum01:fix_cmake_config into a5e1c70 on SRombauts:master.

[SRombauts]

Thanks !