[mlir] Fix DialectRegistry::addExtension compile error

It appears that the DialectRegistry::addExtension template was never
instantiated because it contains an obvious compilation error. Fix it.

Reviewed By: nicolasvasilache

Differential Revision: https://reviews.llvm.org/D123199

mlir/include/mlir/IR/DialectRegistry.h

@@ -176,7 +176,7 @@ public:
   template <typename... ExtensionsT>
   void addExtensions() {
     (void)std::initializer_list<int>{
-        addExtension(std::make_unique<ExtensionsT>())...};
+        (addExtension(std::make_unique<ExtensionsT>()), 0)...};
   }
 
   /// Add an extension function that requires the given dialects.