MAINTAINERS: add linux-api for review of API/ABI changes

This makes it more likely that patch submitters will CC API/ABI changes to
the linux-api list, and tools like get_maintainer.pl will do so
automatically.

Signed-off-by: Josh Triplett <josh@joshtriplett.org>
Acked-by: Steven Rostedt <rostedt@goodmis.org>
Acked-by: Michael Kerrisk <mtk.man-pages@gmail.com>
Cc: Joe Perches <joe@perches.com>
Cc: "David S. Miller" <davem@davemloft.net>
Cc: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>