Added permit to IAToken interface

contracts/interfaces/IAToken.sol

@@ -95,6 +95,27 @@ interface IAToken is IERC20, IScaledBalanceToken, IInitializableAToken {
    **/
   function handleRepayment(address user, uint256 amount) external;
 
+  /**
+   * @dev implements the permit function as for
+   * https://github.com/ethereum/EIPs/blob/8a34d644aacf0f9f8f00815307fd7dd5da07655f/EIPS/eip-2612.md
+   * @param owner The owner of the funds
+   * @param spender The spender
+   * @param value The amount
+   * @param deadline The deadline timestamp, type(uint256).max for max deadline
+   * @param v Signature param
+   * @param s Signature param
+   * @param r Signature param
+   */
+  function permit(
+    address owner,
+    address spender,
+    uint256 value,
+    uint256 deadline,
+    uint8 v,
+    bytes32 r,
+    bytes32 s
+  ) external;
+
   /**
    * @dev Returns the address of the incentives controller contract
    **/

contracts/protocol/tokenization/AToken.sol

@@ -341,7 +341,7 @@ contract AToken is
     uint8 v,
     bytes32 r,
     bytes32 s
-  ) external {
+  ) external override {
     require(owner != address(0), 'INVALID_OWNER');
     //solium-disable-next-line
     require(block.timestamp <= deadline, 'INVALID_EXPIRATION');