[spirv] Bring back accidentally deleted include.

lib/Support/YAMLTraits.cpp

@@ -10,6 +10,7 @@
 #include "llvm/Support/YAMLTraits.h"
 #include "llvm/ADT/SmallString.h"
 #include "llvm/ADT/Twine.h"
+#include "llvm/Support/Casting.h"
 #include "llvm/Support/Errc.h"
 #include "llvm/Support/ErrorHandling.h"
 #include "llvm/Support/Format.h"