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: NJaswinder Singh Rajput <jaswinderrajput@gmail.com>
Signed-off-by: NAndrew Morton <akpm@linux-foundation.org>
Signed-off-by: NLinus Torvalds <torvalds@linux-foundation.org>

mm/vmalloc.c

@@ -25,7 +25,6 @@
 #include <linux/rcupdate.h>
 #include <linux/pfn.h>
 #include <linux/kmemleak.h>
-#include <linux/highmem.h>
 #include <asm/atomic.h>
 #include <asm/uaccess.h>
 #include <asm/tlbflush.h>