define explicit initializer

contracts/token/ERC1155/ERC1155Holder.sol

@@ -10,6 +10,10 @@ import "./ERC1155Receiver.sol";
  * @dev _Available since v3.1._
  */
 contract ERC1155Holder is Initializable, ERC1155Receiver {
+    function initialize() public initializer {
+        ERC1155Receiver.initialize();
+    }
+
     function onERC1155Received(address, address, uint256, uint256, bytes memory) public returns (bytes4) {
         return this.onERC1155Received.selector;
     }