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/compiler.h b/src/compiler.h
index bafd9d2..8facc96 100644
--- a/src/compiler.h
+++ b/src/compiler.h
@@ -21,13 +21,13 @@
 #include <string>
 #include <vector>
 
+#include "base/mutex.h"
 #include "compiled_class.h"
 #include "compiled_method.h"
 #include "dex_cache.h"
 #include "dex_file.h"
 #include "instruction_set.h"
 #include "invoke_type.h"
-#include "mutex.h"
 #include "oat_file.h"
 #include "object.h"
 #include "runtime.h"