gutf8: Add various missing (transfer) annotations

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

Fixes: #872
10 jobs for 872-ucs-annotations in 31 minutes and 38 seconds (queued for 3 seconds)
Status Job ID Name Coverage
  Build
passed #333339
cross-android_api21_arm64

00:02:15

passed #333340
cross-android_api28_arm64

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passed #333341
cross-mingw64

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passed #333337
debian-stable-x86_64

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passed #333336
fedora-x86_64

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passed #333338
G_DISABLE_ASSERT

00:03:17

passed #333342
win32
msys2-mingw32

00:08:58

passed #333343
win32
vs2017-x64

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  Coverage
passed #333344
coverage

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77.8%
 
  Analysis
passed #333345
scan-build

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