ir_to_mesa: Move some things outside the 'extern "C"' blocks

ir_to_mesa: Move some things outside the 'extern "C"' blocks

Having a few of these includes or forward declarations inside the
'extern "C"' block can cause problems later.  Specifically, it
prevents C++ linkage functions from being added to ir_to_mesa.h and
makes G++ angry if 'struct foo' is seen both inside and outside an
'extern "C"'.

Signed-off-by: Ian Romanick <ian.d.romanick@intel.com>
Reviewed-by: Kenneth Graunke <kenneth@whitecape.org>