mdo: Avoid relying on header pollution

The uses of PAGE_SIZE and roundup2() require param.h.

MFC after:	1 week

usr.bin/mdo/mdo.c

@@ -10,9 +10,9 @@
  * Foundation.
  */
 
+#include <sys/param.h>
 #include <sys/errno.h>
 #include <sys/limits.h>
-#include <sys/types.h>
 #include <sys/ucred.h>
 
 #include <assert.h>