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.cc b/src/jni_internal.cc
index d2c8fc4..43532c2 100644
--- a/src/jni_internal.cc
+++ b/src/jni_internal.cc
@@ -22,12 +22,12 @@
 #include <utility>
 #include <vector>
 
+#include "base/mutex.h"
 #include "class_linker.h"
 #include "class_loader.h"
 #include "invoke_arg_array_builder.h"
 #include "jni.h"
 #include "logging.h"
-#include "mutex.h"
 #include "object.h"
 #include "object_utils.h"
 #include "runtime.h"