[mlir][nvgpu] Ignore -Wunused-function in NVGPUDialect.cpp (NFC)

In file included from /Users/jiefu/llvm-project/mlir/lib/Dialect/NVGPU/IR/NVGPUDialect.cpp:363:
/Users/jiefu/llvm-project/build-Release/tools/mlir/include/mlir/Dialect/NVGPU/IR/NVGPUAttrDefs.cpp.inc:22:36: error: unused function 'generatedAttributeParser' [-Werror,-Wunused-function]
static ::mlir::OptionalParseResult generatedAttributeParser(::mlir::AsmParser &parser, ::llvm::StringRef *mnemonic, ::mlir::Type type, ::mlir::Attribute &value) {
                                   ^
/Users/jiefu/llvm-project/build-Release/tools/mlir/include/mlir/Dialect/NVGPU/IR/NVGPUAttrDefs.cpp.inc:46:30: error: unused function 'generatedAttributePrinter' [-Werror,-Wunused-function]
static ::mlir::LogicalResult generatedAttributePrinter(::mlir::Attribute def, ::mlir::AsmPrinter &printer) {
                             ^
2 errors generated.