Unblock buildbot.

llvm-svn: 186133
diff --git a/lldb/source/Symbol/ClangASTContext.cpp b/lldb/source/Symbol/ClangASTContext.cpp
index b5c658d..5ba9c6a 100644
--- a/lldb/source/Symbol/ClangASTContext.cpp
+++ b/lldb/source/Symbol/ClangASTContext.cpp
@@ -70,9 +70,10 @@
 #include "lldb/Target/Process.h"
 #include "lldb/Target/ObjCLanguageRuntime.h"
 
-
 #include <stdio.h>
 
+#include <mutex>
+
 using namespace lldb;
 using namespace lldb_private;
 using namespace llvm;