Make getApproved constant view

Author: fulldecent

EIPS/eip-721.md

@@ -132,7 +132,7 @@ interface ERC721 /* is ERC165  */ {
     /// @dev Throws if `_tokenId` is not a valid NFT
     /// @param _tokenId The NFT to find the approved address for
     /// @return The approved address for this NFT, or the zero address if there is none
-    function getApproved(uint256 _tokenId) external returns (address);
+    function getApproved(uint256 _tokenId) external view returns (address);
 
     /// @notice Query if an address is an authorized operator for another address
     /// @param _owner The address that owns the NFTs