fix build gcc 4.3

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@56965 91177308-0d34-0410-b5e6-96231b3b80d8
diff --git a/lib/CodeGen/PBQP.cpp b/lib/CodeGen/PBQP.cpp
index ade9e7c..00ca2a8 100644
--- a/lib/CodeGen/PBQP.cpp
+++ b/lib/CodeGen/PBQP.cpp
@@ -15,6 +15,7 @@
 
 #include <limits>
 #include <cassert>
+#include <cstring>
 
 #include "PBQP.h"