x86: fix ds.c build error

fix:
arch/x86/kernel/ds.c: In function ‘ds_allocate_buffer':
arch/x86/kernel/ds.c:339: error: implicit declaration of function ‘PAGE_ALIGN'
Signed-off-by: Ingo Molnar <mingo@elte.hu>

arch/x86/kernel/ds.c

@@ -29,6 +29,7 @@
 #include <linux/string.h>
 #include <linux/slab.h>
 #include <linux/sched.h>
+#include <linux/mm.h>
 
 
 /*