mm: fix implicit stat.h usage in dmapool.c

The removal of the implicitly everywhere module.h and its child includes
will reveal this implicit stat.h usage:

mm/dmapool.c:108: error: ‘S_IRUGO’ undeclared here (not in a function)
Signed-off-by: NPaul Gortmaker <paul.gortmaker@windriver.com>

mm/dmapool.c

@@ -32,6 +32,7 @@
 #include <linux/poison.h>
 #include <linux/sched.h>
 #include <linux/slab.h>
+#include <linux/stat.h>
 #include <linux/spinlock.h>
 #include <linux/string.h>
 #include <linux/types.h>