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/monitor.cc b/src/monitor.cc
index 1fbda87..d58f462 100644
--- a/src/monitor.cc
+++ b/src/monitor.cc
@@ -18,9 +18,9 @@
 
 #include <vector>
 
+#include "base/mutex.h"
 #include "class_linker.h"
 #include "dex_instruction.h"
-#include "mutex.h"
 #include "object.h"
 #include "object_utils.h"
 #include "scoped_thread_state_change.h"