[mlir] replace class with struct in a forward declaration

Silence the warning.

Differential Revision: https://reviews.llvm.org/D113018

Author: ftynse

mlir/include/mlir/Dialect/Linalg/Transforms/BufferizableOpInterface.h

@@ -18,7 +18,7 @@ namespace mlir {
 class BlockAndValueMapping;
 
 namespace linalg {
-class AllocationCallbacks;
+struct AllocationCallbacks;
 class BufferizationAliasInfo;
 
 /// Specify fine-grain relationship between buffers to enable more analysis.