ringct: add operator!= for `key`

Part of upstreaming Seraphis/Carrot

Co-authored-by: j-berman <justinberman@protonmail.com>

src/ringct/rctTypes.h

@@ -84,6 +84,7 @@ namespace rct {
             return bytes[i];
         }
         bool operator==(const key &k) const { return !crypto_verify_32(bytes, k.bytes); }
+        bool operator!=(const key &k) const { return crypto_verify_32(bytes, k.bytes); }
         unsigned char bytes[32];
     };
     typedef std::vector<key> keyV; //vector of keys