Fix the build with gcc 4.3 as the host, patch by Zhongxing Xu!

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@55115 91177308-0d34-0410-b5e6-96231b3b80d8

include/llvm/ADT/APInt.h

@@ -21,6 +21,7 @@
 #include <cstring>
 #include <iosfwd>
 #include <string>
+#include <cstring>
 
 namespace llvm {
   class Serializer;