Move mutex.h into base/mutex.h.

We don't actually need any hackery to get the right mutex.h any
more, but since we planned on having the google3-like stuff
in google3-like locations, this is an easy one to fix.

Change-Id: Ie27464ebbc208a6f4e694a97cf26b1cee0737009
diff --git a/src/well_known_classes.h b/src/well_known_classes.h
index fd8f19b..f2c479c 100644
--- a/src/well_known_classes.h
+++ b/src/well_known_classes.h
@@ -17,8 +17,8 @@
 #ifndef ART_SRC_WELL_KNOWN_CLASSES_H_
 #define ART_SRC_WELL_KNOWN_CLASSES_H_
 
+#include "base/mutex.h"
 #include "jni.h"
-#include "mutex.h"
 
 namespace art {