mesa: declare pipe_texture in st_public.h

mesa: declare pipe_texture in st_public.h

Fixes:
In file included from nouveau_context_dri.c:6:
../../../../../../src/mesa/state_tracker/st_public.h:87: warning:
'struct pipe_texture' declared inside parameter list
../../../../../../src/mesa/state_tracker/st_public.h:87: warning: its
scope is only this definition or declaration, which is probably not what
you want
../../../../../../src/mesa/state_tracker/st_public.h:104: warning:
'struct pipe_texture' declared inside parameter list

Signed-off-by: Pekka Paalanen <pq@iki.fi>