Add include guards to prfchwintrin.h.

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@181513 91177308-0d34-0410-b5e6-96231b3b80d8

lib/Headers/prfchwintrin.h

@@ -25,6 +25,9 @@
 #error "Never use <prfchwintrin.h> directly; include <x86intrin.h> or <mm3dnow.h> instead."
 #endif
 
+#ifndef __PRFCHWINTRIN_H
+#define __PRFCHWINTRIN_H
+
 #if defined(__PRFCHW__) || defined(__3dNOW__)
 static __inline__ void __attribute__((__always_inline__, __nodebug__))
 _m_prefetchw(void *__P)
@@ -32,3 +35,5 @@ _m_prefetchw(void *__P)
   __builtin_prefetch (__P, 1, 3 /* _MM_HINT_T0 */);
 }
 #endif
+
+#endif /* __PRFCHWINTRIN_H */