Try to fix NetBSD buildbot breakage introduced in D57463.

By including the header file in the source.



git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@355202 91177308-0d34-0410-b5e6-96231b3b80d8

lib/Transforms/Instrumentation/InstrOrderFile.cpp

@@ -31,6 +31,7 @@
 #include "llvm/Transforms/Instrumentation/InstrOrderFile.h"
 #include <fstream>
 #include <map>
+#include <mutex>
 #include <set>
 #include <sstream>