Fix .gitignore: stop tracking ignored files

backend/lib/openzeppelin-contracts/contracts/mocks/token/ERC721URIStorageMock.sol

@@ -0,0 +1,17 @@
+// SPDX-License-Identifier: MIT
+
+pragma solidity ^0.8.0;
+
+import "../../token/ERC721/extensions/ERC721URIStorage.sol";
+
+abstract contract ERC721URIStorageMock is ERC721URIStorage {
+    string private _baseTokenURI;
+
+    function _baseURI() internal view virtual override returns (string memory) {
+        return _baseTokenURI;
+    }
+
+    function setBaseURI(string calldata newBaseTokenURI) public {
+        _baseTokenURI = newBaseTokenURI;
+    }
+}