fix(moonnirds): moonbird nesting open view function fix (#313)

Author: WalidOfNow

contracts/dependencies/erc721-collections/IMoonBird.sol

@@ -5,7 +5,7 @@ pragma solidity 0.8.10;
     @title MoonBirds contract interface
  */
 interface IMoonBirdBase {
-    function nestingOpen() external returns(bool);
+    function nestingOpen() external view returns(bool);
 
     function toggleNesting(uint256[] calldata tokenIds) external;
 

contracts/protocol/tokenization/NTokenMoonBirds.sol

@@ -123,7 +123,7 @@ contract NTokenMoonBirds is NToken, IMoonBirdBase {
         @dev an additional function that is custom to MoonBirds reserve.
         This function check if nesting is open for the underlying tokens
     */
-    function nestingOpen() external returns (bool) {
+    function nestingOpen() external view returns (bool) {
         return IMoonBird(_underlyingAsset).nestingOpen();
     }
 }