pkcs11-tool.c: fix compiler warning

pkcs11-tool.c:2992:1: warning: control reaches end of non-void function
      [-Wreturn-type]
}
^

src/tools/pkcs11-tool.c

@@ -2988,6 +2988,7 @@ derive_ec_key(CK_SESSION_HANDLE session, CK_OBJECT_HANDLE key, CK_MECHANISM_TYPE
 	return newkey;
 #else
 	util_fatal("Derive EC key not supported");
+	return 0;
 #endif /* ENABLE_OPENSSL  && !OPENSSL_NO_EC && !OPENSSL_NO_ECDSA */
 }