build: install gobject-introspection with MSYS2 when cross-building…

… for Windows.

Though it's useless for actually building the GIR files, we still need
this package now, for building script-fu with introspection abilities,
to generate GIMP and GEGL enums.
See the 2 previous commits for more information.
15 jobs for !523 with nielsdg/script-fu-use-girepository in 101 minutes and 26 seconds (queued for 1 second)
latest merge request