Fix compiler warnings gallium
This allows gallium builds to get closer to being built with -Werror
on my machine. However there are C++ warnings with SWR such as:
clearing an object of type 'struct ' with no trivial copy-assignment; use assignment or value-initialization instead
Which I'm not sure how they should be fixed, so I probably won't pursue enabling -Werror
for meson-gallium any further unless someone resolves those. I've filed a SWR bug for this #3227 (closed)