[MLIR] Add missing include (NFC)

Needed for libstdc++ 15 compatibility.

Author: nikic

mlir/include/mlir/Target/SPIRV/Serialization.h

@@ -14,6 +14,7 @@
 #define MLIR_TARGET_SPIRV_SERIALIZATION_H
 
 #include "mlir/Support/LLVM.h"
+#include <cstdint>
 
 namespace mlir {
 class MLIRContext;