gmain.c: Fix comment about Y2038 safety of g_get_real_time()

This comment was correct until commit adf1f98f, when the `GTimeVal`
which the result was put into (introducing the Y2038-unsafety) was
dropped.

The adjustment and scaling of the `FILETIME` should not make it
Y2038-unsafe: the maximum `FILETIME` is 2^64-1. Subtracting the epoch
adjustment and dividing by 10 gives the timestamp 1833029933770955161,
which is in June 58086408216 (at just after 3am UTC). I think that’s
enough time to be going on with.

Signed-off-by: Philip Withnall <withnall@endlessm.com>

Helps: #1438
12 jobs for 1438-deprecate-gtimeval in 48 minutes and 8 seconds (queued for 2 seconds)
Status Job ID Name Coverage
  Build
passed #371468
cross-android_api21_arm64

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passed #371469
cross-android_api28_arm64

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passed #371470
cross-mingw64

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passed #371466
debian-stable-x86_64

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passed #371465
fedora-x86_64

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passed #371467
G_DISABLE_ASSERT

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passed #371503
win32
msys2-mingw32

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passed #371502
win32
vs2017-x64

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canceled #371471
win32
msys2-mingw32

canceled #371472
win32
vs2017-x64

 
  Coverage
passed #371473
coverage

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77.9%
 
  Analysis
passed #371474
scan-build

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