meson: Fix build with -Dopenjpeg=disabled

tests/check/ uses the openjpeg_dep variable
unconditionally, and the subdir_done() is useless anyway, since the
plugin is only built if openjpeg_dep.found() is true. Fixes:

..\tests\check\ ERROR: Unknown variable "openjpeg_dep".

In particular, this fixes the build on UWP since we disable openjpeg
explicitly in Cerbero when building for UWP.
33 jobs for !2545 with fix-openjpeg-disabled-build in 43 seconds (queued for 10 seconds)
latest merge request