meson: Unkonw variables while disabling msdk plugin

Submitted by Jordan Petridis

Link to original bug (#797354)

Description

While building with auto_features=enabled -Dmsdk=disabled I encountered the following error.

'tests/check/meson.build:18:0: ERROR: Unknown variable "have_msdk".'

same error was also thrown later for "msdk_dep".

This happened right after 55134df5 which to my understanding added an early exit before the variables where declared.

I am not sure if the following patch is the proper way to fix it, using auto-feature magic in the test too would probably be cleaner but I did not managed to figure out if its possible.

Tested the following patch in gnome-build-meta. https://gitlab.gnome.org/GNOME/gnome-build-meta/commit/8915cb51a25559163e2f1e6736742d8c242acf5f