vk: Drop -Wextra

vk: Drop -Wextra

Eliminates lots of warnings due to anv_meta.c's inclusion of nir.h.

I like the extra warnings, and they should probably get fixed. However,
git-grep reveals that no other Mesa directory uses -Wextra. Building
Vulkan produces a lot of compiler warnings from core Mesa headers that
no other Mesa developer sees, and hence no other Mesa developer will
fix.