[mlir] Fix a warning

This patch fixes:

  mlir/lib/Dialect/Linalg/IR/LinalgInterfaces.cpp:906:11: error:
  enumeration value 'EmptyConvolvedDims' not handled in switch
  [-Werror,-Wswitch]

with a workaround.  I've notified the author of the new enum value in
#102087.

Author: kazutakahirata

mlir/lib/Dialect/Linalg/IR/LinalgInterfaces.cpp

@@ -918,6 +918,8 @@ mlir::linalg::detail::getMatchConvolutionMessage(MatchConvolutionResult res) {
     return "expected all iterators used to access outputs to be parallel";
   case MatchConvolutionResult::NonOutputDimNotReduction:
     return "expected all iterators not used to access outputs to be reduction";
+  case MatchConvolutionResult::EmptyConvolvedDims:
+    return "FIXME";
   case MatchConvolutionResult::Success:
     return "";
   }