Added dmm_config header guard.

dmm_config.h.in

@@ -1,2 +1,8 @@
+#ifndef DMM_CONFIG_H
+#define DMM_CONFIG_H
+
 #cmakedefine HAVE_LOCKS
 #cmakedefine WITH_MEMORY_SPACE_AWARENESS
+
+#endif /* DMM_CONFIG_H */
+