Fix extraDeclarations for AllGather (#2262)

Missing the common declarations.
diff --git a/stablehlo/dialect/StablehloOps.td b/stablehlo/dialect/StablehloOps.td
index a4d2ec4..2521117 100644
--- a/stablehlo/dialect/StablehloOps.td
+++ b/stablehlo/dialect/StablehloOps.td
@@ -1349,7 +1349,7 @@
   let results = (outs HLO_Tensor);
   let hasVerifier = 1;
 
-  let extraClassDeclaration = [{
+  let extraClassDeclaration = commonClassDeclaration # [{
     /// Interface method for ConditionallySpeculatable.
     mlir::Speculation::Speculatability getSpeculatability();
   }];