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.
This fixes the cerbero build on Windows when the Intel Media SDK is installed but we're building with MinGW so the plugin can't be enabled.