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.