i965: forward-declare struct brw_context in brw_reg.h

i965: forward-declare struct brw_context in brw_reg.h

Commit 54e91e7420 introduced a function declaration that uses
brw_context. While brw_context tends to get included in most files, it
is not when compiling intel_asm_annotation.c resulting in the following
warning:

In file included from brw_shader.h:25:0,
                 from brw_cfg.h:32,
                 from intel_asm_annotation.c:24:
brw_reg.h:122:39: warning: 'struct brw_context' declared inside
parameter list [enabled by default]
brw_reg.h:122:39: warning: its scope is only this definition or
declaration, which is probably not what you want [enabled by default]

Add a forward-declaration for struct brw_context to avoid the issue.

Signed-off-by: Ilia Mirkin <imirkin@alum.mit.edu>
Reviewed-by: Kenneth Graunke <kenneth@whitecape.org>