Fix installing gocode on Windows

[arp242]

Was missing a space in composing the command, so this would get run:

go get -v -u -ldflags -H=windowsgui"github.com/nsf/gocode"

[codecov-io]

Codecov Report

❗ No coverage uploaded for pull request base (master@2a42722). Click here to learn what that means.
The diff coverage is 0%.

@@            Coverage Diff            @@
##             master    #1606   +/-   ##
=========================================
  Coverage          ?   14.78%           
=========================================
  Files             ?       53           
  Lines             ?     4152           
  Branches          ?        0           
=========================================
  Hits              ?      614           
  Misses            ?     3538           
  Partials          ?        0

Flag	Coverage Δ
#nvim	5.05% <0%> (?)
#vim74	14.71% <0%> (?)
#vim80	14.47% <0%> (?)

Impacted Files	Coverage Δ
plugin/go.vim	29.26% <0%> (ø)

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