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/locks.cc b/src/locks.cc
index 0422aff..27b9d4b 100644
--- a/src/locks.cc
+++ b/src/locks.cc
@@ -16,7 +16,7 @@
 
 #include "locks.h"
 
-#include "mutex.h"
+#include "base/mutex.h"
 
 namespace art {