[Support] System include SipHash.h

Match the SYSTEM directive to this header which was introduced by
7f3afab, because a regular include may skip system directories.

Author: guy-david

llvm/lib/Support/SipHash.cpp

@@ -12,13 +12,13 @@
 //===----------------------------------------------------------------------===//
 
 #include "llvm/Support/SipHash.h"
-#include "siphash/SipHash.h"
 #include "llvm/ADT/ArrayRef.h"
 #include "llvm/ADT/StringExtras.h"
 #include "llvm/ADT/StringRef.h"
 #include "llvm/Support/Debug.h"
 #include "llvm/Support/Endian.h"
 #include <cstdint>
+#include <siphash/SipHash.h>
 
 using namespace llvm;
 using namespace support;