MAINTAINERS: mm: add additional include files to listing

Add gfp.h, mmzone.h, memory_hotplug.h & vmalloc.h to the "MEMORY
MANAGMENT" section so scripts/get_maintainer.pl can do a better job of
making recommendations.

Signed-off-by: Cody P Schafer <cody@linux.vnet.ibm.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>