mesa: move ADD_POINTERS to macros.h

mesa: move ADD_POINTERS to macros.h

I'm not really sure where else to put it. Since imports.h only has two
things left in it (neither of which are abstractions for smoothing away
libc differences) I'd like to get them out of there. macros.h is the
only place I can think of to put this macro.

Reviewed-by: Marek Olšák <marek.olsak@amd.com>
Reviewed-by: Kristian H. Kristensen <hoegsberg@google.com>
Reviewed-by: Matt Turner <mattst88@gmail.com>
Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/3024>