meson: Fix automagic build of msdk plugin

When building the msdk plugin even if libmfx is found, unless the
plugin is explicitly enabled we should not error out if msdk
dependencies are not found.

Also give an error message when we don't build the plugin on Windows
because we're not building with MSVC.
8 jobs for fix-msdk-auto in 18 minutes and 25 seconds (queued for 3 seconds)
Status Name Job ID Coverage
  Preparation
passed gst indent #85237

00:00:48

passed manifest #85236

00:00:40

 
  Build
passed build fedora x86_64 #85238

00:04:19

 
  Test
passed check fedora #85239

00:03:35

 
  Full Builds
passed build android arm64 api28 #85240

00:03:43

passed build cerbero cross win32 #85242

00:09:41

passed build cerbero cross win64 #85243

00:07:40

passed build cerbero fedora x86_64 #85241

00:07:56