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/jni_internal.h b/src/jni_internal.h
index 606887c..6b597ec 100644
--- a/src/jni_internal.h
+++ b/src/jni_internal.h
@@ -20,9 +20,9 @@
 #include "jni.h"
 
 #include "base/macros.h"
+#include "base/mutex.h"
 #include "heap.h"
 #include "indirect_reference_table.h"
-#include "mutex.h"
 #include "reference_table.h"
 #include "runtime.h"