cmake/plugins: Enable -Wswitch on GCC

This warning is already enabled for Clang which causes issue for things
that are usually built with MSVC or GCC (e.g. non-macOS plugins).

obs-nvenc switch warning is also fixed in this commit.
obs-qsv11 has switch warning set to not become errors.
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Gitea Version: 1.28.0+dev-477-g8b6ad49a5f