MAINTAINERS: add uapi directories to API/ABI section

Let's add include/uapi/ and arch/*/include/uapi/ to API/ABI section, so
that for patches modifying them, get_maintainers.pl suggests CCing
linux-api@ so people don't forget.

Link: https://lkml.kernel.org/r/20210217174745.13591-1-vbabka@suse.cz


Signed-off-by: Vlastimil Babka <vbabka@suse.cz>
Reported-by: David Hildenbrand <david@redhat.com>
Cc: Michael Kerrisk <mtk.manpages@gmail.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>