nir: Try to warn when C99 extensions are used in nir headers.

nir: Try to warn when C99 extensions are used in nir headers.

Ideally we'd have nir.h being included with -Wpedantic too, but it fails
with:

src/compiler/nir/nir.h:754:20: warning: ISO C++ forbids zero-size array ‘src’ [-Wpedantic]
    nir_alu_src src[];
                    ^
In file included from src/compiler/nir/glsl_to_nir.cpp:42:0:
src/compiler/nir/nir.h:919:16: warning: ISO C++ forbids zero-size array ‘src’ [-Wpedantic]
    nir_src src[];

Reviewed-by: Connor Abbott <cwabbott0@gmail.com>