Fix .gitignore: stop tracking ignored files

backend/lib/openzeppelin-contracts/contracts/utils/Counters.sol

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+// SPDX-License-Identifier: MIT
+// OpenZeppelin Contracts v4.4.1 (utils/Counters.sol)
+
+pragma solidity ^0.8.0;
+
+/**
+ * @title Counters
+ * @author Matt Condon (@shrugs)
+ * @dev Provides counters that can only be incremented, decremented or reset. This can be used e.g. to track the number
+ * of elements in a mapping, issuing ERC721 ids, or counting request ids.
+ *
+ * Include with `using Counters for Counters.Counter;`
+ */
+library Counters {
+    struct Counter {
+        // This variable should never be directly accessed by users of the library: interactions must be restricted to
+        // the library's function. As of Solidity v0.5.2, this cannot be enforced, though there is a proposal to add
+        // this feature: see https://github.com/ethereum/solidity/issues/4637
+        uint256 _value; // default: 0
+    }
+
+    function current(Counter storage counter) internal view returns (uint256) {
+        return counter._value;
+    }
+
+    function increment(Counter storage counter) internal {
+        unchecked {
+            counter._value += 1;
+        }
+    }
+
+    function decrement(Counter storage counter) internal {
+        uint256 value = counter._value;
+        require(value > 0, "Counter: decrement overflow");
+        unchecked {
+            counter._value = value - 1;
+        }
+    }
+
+    function reset(Counter storage counter) internal {
+        counter._value = 0;
+    }
+}