Fix .gitignore: stop tracking ignored files

backend/lib/openzeppelin-contracts/contracts/token/ERC20/presets/ERC20PresetFixedSupply.sol

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+// SPDX-License-Identifier: MIT
+// OpenZeppelin Contracts (last updated v4.9.0) (token/ERC20/presets/ERC20PresetFixedSupply.sol)
+pragma solidity ^0.8.0;
+
+import "../extensions/ERC20Burnable.sol";
+
+/**
+ * @dev {ERC20} token, including:
+ *
+ *  - Preminted initial supply
+ *  - Ability for holders to burn (destroy) their tokens
+ *  - No access control mechanism (for minting/pausing) and hence no governance
+ *
+ * This contract uses {ERC20Burnable} to include burn capabilities - head to
+ * its documentation for details.
+ *
+ * _Available since v3.4._
+ *
+ * _Deprecated in favor of https://wizard.openzeppelin.com/[Contracts Wizard]._
+ */
+contract ERC20PresetFixedSupply is ERC20Burnable {
+    /**
+     * @dev Mints `initialSupply` amount of token and transfers them to `owner`.
+     *
+     * See {ERC20-constructor}.
+     */
+    constructor(string memory name, string memory symbol, uint256 initialSupply, address owner) ERC20(name, symbol) {
+        _mint(owner, initialSupply);
+    }
+}