libgimp: sync autotools and meson to the same list of introspectable…

… headers.
gimpitem.h was missing, as noted (again!) by Massimo, which resulted
into introspected plug-ins errors (breaking parenting of classes).
5 jobs for master in 25 minutes and 11 seconds
Status Job ID Name Coverage
  Dependencies
passed #461954
deps-git

00:02:53

passed #461955
deps-x86_64-w64-mingw32

00:11:49

 
  Gimp
passed #461956
gimp-autotools

00:13:21

passed #461957
gimp-meson

00:10:39

passed #461958
gimp-x86_64-w64-mingw32-meson

00:08:36