ART: Move to using locks.h instead of mutex.h

When only annotating lock requirements, use locks.h.

Bug: 119869270
Test: mmma art
Change-Id: I1608b03254712feff0072ebad012c3af0cc3dda4
diff --git a/runtime/obj_ptr.h b/runtime/obj_ptr.h
index efbb66f..9e2ee29 100644
--- a/runtime/obj_ptr.h
+++ b/runtime/obj_ptr.h
@@ -21,8 +21,8 @@
 #include <type_traits>
 
 #include "base/globals.h"
+#include "base/locks.h"  // For Locks::mutator_lock_.
 #include "base/macros.h"
-#include "base/mutex.h"  // For Locks::mutator_lock_.
 
 // Always inline ObjPtr methods even in debug builds.
 #define OBJPTR_INLINE __attribute__ ((always_inline))