libgimp: reduce GObject Introspection warnings.

Thanks to Massimo for helping:

> To reduce the number of warnings down to 2, it is necessary to list
> gimptypes.h and gimpenums.h right after gimp.h and gimpui.h
> gimpuitypes.h at the start of libgimpui_headers_introspectable like
> they are in the corresponding list in Makefile.gi.
>
> Their order evidently matters

See #4018.
Some stuff are still wrong, but this is a first step in the right
direction.
5 jobs for master in 10 minutes and 34 seconds (queued for 1 second)
Status Job ID Name Coverage
  Dependencies
passed #461934
deps-git

00:05:15

failed #461935
deps-x86_64-w64-mingw32

00:10:34

 
  Gimp
skipped #461936
gimp-autotools
skipped #461937
gimp-meson
skipped #461938
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 2691 matching files
_deps/: found 584 matching files

Uploading artifacts to coordinator... ok
id=461935 responseStatus=201 Created token=dzFqsgD5
ERROR: Job failed: exit code 1