plug-ins: enable screenshot for non x11 targets in meson build.

This also removes screenshot as an option for the meson build
since the autotools build doesn't have that option either.
12 jobs for screenshot in 36 minutes and 55 seconds (queued for 1 second)
Status Job ID Name Coverage
  Prepare
passed #836688
build-image

00:02:38

passed #836690
build-image-win32

00:02:04

passed #836689
build-image-win64

00:02:05

 
  Dependencies
passed #836691
deps-debian/testing

00:02:33

passed #836693
deps-win32

00:17:28

passed #836692
deps-win64

00:09:21

 
  Gimp
passed #836694
gimp-debian/testing-autotools

00:09:49

passed #836696
gimp-debian/testing-clang

00:05:32

passed #836695
gimp-debian/testing-meson

00:05:43

passed #836698
gimp-win32

00:17:19

passed #836697
gimp-win64

00:13:25

 
  Analysis
passed #836699
cppcheck

00:02:08