mm: includecheck fix: vmalloc.c

fix the following 'make includecheck' warning:

  mm/vmalloc.c: linux/highmem.h is included more than once.

Signed-off-by: Jaswinder Singh Rajput <jaswinderrajput@gmail.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>