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_llvm/compilation_unit.h b/src/compiler_llvm/compilation_unit.h
index 0b40388..5fb0c0c 100644
--- a/src/compiler_llvm/compilation_unit.h
+++ b/src/compiler_llvm/compilation_unit.h
@@ -17,7 +17,7 @@
 #ifndef ART_SRC_COMPILER_LLVM_COMPILATION_UNIT_H_
 #define ART_SRC_COMPILER_LLVM_COMPILATION_UNIT_H_
 
-#include "../mutex.h"
+#include "base/mutex.h"
 #include "globals.h"
 #if defined(ART_USE_DEXLANG_FRONTEND)
 # include "greenland/dex_lang.h"