i965: Make brw_{shader,vec4}.h safe to include from C.

i965: Make brw_{shader,vec4}.h safe to include from C.

The patch that follows will move the definition of struct
brw_vec4_prog_key from brw_vs.h to brw_vec4.h, making it necessary for
brw_vs.h to include brw_vec4.h (because brw_vs.h defines struct
brw_vs_prog_key, which contains brw_vec4_prog_key as a member).  Since
brw_vs.h is included from C source files, that means that brw_vec4.h
will need to be safe to include from C.  Same for brw_shader.h, since
it is included by brw_vec4.h.

Reviewed-by: Ian Romanick <ian.d.romanick@intel.com>
Reviewed-by: Kenneth Graunke <kenneth@whitecape.org>
Reviewed-by: Matt Turner <mattst88@gmail.com>