i965: Make precompile functions accessible from C.

i965: Make precompile functions accessible from C.

Previously, the prototypes for brw_vs/gs/fs_precompile were scattered
between brw_vs.h (C), brw_gs.h (C), and brw_fs.h (C++ only).  Also,
brw_fs_precompile had C++ linkage, while the others were C.

This patch moves all the prototypes to a central location (brw_shader.h)
and makes brw_fs_precompile have C linkage.

Signed-off-by: Kenneth Graunke <kenneth@whitecape.org>
Reviewed-by: Kristian Høgsberg <krh@bitplanet.net>
Reviewed-by: Matt Turner <mattst88@gmail.com>