[mlir] Fix a missing override warning

This patch fixes:

  mlir/lib/Dialect/Tosa/Transforms/TosaOptionalDecompositions.cpp:28:8:
  error: 'runOnFunction' overrides a member function but is not marked
  'override' [-Werror,-Wsuggest-override]

mlir/lib/Dialect/Tosa/Transforms/TosaOptionalDecompositions.cpp

@@ -25,7 +25,7 @@ namespace {
 
 struct TosaOptionalDecompositions
     : public TosaOptionalDecompositionsBase<TosaOptionalDecompositions> {
-  void runOnFunction() {
+  void runOnFunction() override {
     auto *ctx = &getContext();
     RewritePatternSet patterns(ctx);
     auto func = getFunction();