meson: va: Skip configuration on non-linux environment
VA plugin is linux-only plugin, so we can skip it earlier. Note that this plugin is making use of libdrm meson fallback, which is unusable on the other platforms such as Windows
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VA plugin is linux-only plugin, so we can skip it earlier. Note that this plugin is making use of libdrm meson fallback, which is unusable on the other platforms such as Windows