app, libgimp: fix more unused function warning (Win32).

Similar to commit 9b328167. Apparently the same function is duplicated
into app/.
Also add some comments to detail a bit more where this Win32 different
comes from (i.e. bug 359538 on Bugzilla, commit eac61e1e).

Moreover I tested the same steps as comment 4 in bug 359538 on master
code. The same hanging issue still exists with GTK+ 3.24.1 (in a Windows
VM at least). I have not investigated further, just wanted to check if
this ifndef could not simply be removed by now. Apparently not.
5 jobs for master in 15 minutes and 48 seconds (queued for 1 second)
Status Job ID Name Coverage
  Dependencies
passed #462122
deps-git

00:04:41

failed #462123
deps-x86_64-w64-mingw32

00:15:48

 
  Gimp
skipped #462124
gimp-autotools
skipped #462125
gimp-meson
skipped #462126
gimp-x86_64-w64-mingw32-meson
 
Name Stage Failure
failed
deps-x86_64-w64-mingw32 Dependencies
Tell your friend boy Greg T.
that you were standing at the crossroads.
I believe you were sinking down.


Uploading artifacts...
.local/: found 6043 matching files
_deps/: found 2938 matching files

Uploading artifacts to coordinator... ok
id=462123 responseStatus=201 Created token=zyNtCwsg
ERROR: Job failed: exit code 1