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/thread.cc b/src/thread.cc
index fb7aa5a..aa13ad2 100644
--- a/src/thread.cc
+++ b/src/thread.cc
@@ -28,6 +28,7 @@
 #include <iostream>
 #include <list>
 
+#include "base/mutex.h"
 #include "class_linker.h"
 #include "class_loader.h"
 #include "cutils/atomic.h"
@@ -37,7 +38,6 @@
 #include "heap.h"
 #include "jni_internal.h"
 #include "monitor.h"
-#include "mutex.h"
 #include "oat/runtime/context.h"
 #include "object.h"
 #include "object_utils.h"