[mlir] Fix a warning

This patch fixes:

  mlir/lib/Target/LLVMIR/AttrKindDetail.h:65:1: error: unused function
  'getAttrNameToKindMapping' [-Werror,-Wunused-function]

Author: kazutakahirata

mlir/lib/Target/LLVMIR/AttrKindDetail.h

@@ -61,7 +61,8 @@ getAttrKindToNameMapping() {
 
 /// Returns a dense map from LLVM attribute name to their kind in LLVM IR
 /// dialect.
-static llvm::DenseMap<llvm::StringRef, llvm::Attribute::AttrKind>
+[[maybe_unused]] static llvm::DenseMap<llvm::StringRef,
+                                       llvm::Attribute::AttrKind>
 getAttrNameToKindMapping() {
   static auto attrNameToKindMapping = []() {
     llvm::DenseMap<llvm::StringRef, llvm::Attribute::AttrKind> nameKindMap;