mm_types_task.h: Trim dependencies

more sched.h header dependency trimming
Signed-off-by: Kent Overstreet <kent.overstreet@linux.dev>

include/linux/mm_types_task.h

@@ -9,9 +9,6 @@
  */
 
 #include <linux/types.h>
-#include <linux/threads.h>
-#include <linux/atomic.h>
-#include <linux/cpumask.h>
 
 #include <asm/page.h>
 
@@ -36,6 +33,8 @@ enum {
 	NR_MM_COUNTERS
 };
 
+struct page;
+
 struct page_frag {
 	struct page *page;
 #if (BITS_PER_LONG > 32) || (PAGE_SIZE >= 65536)