Fix error format in gio/gunixconnection.c

Fixes:

gunixconnection.c: In function ‘g_unix_connection_receive_fd’:
gunixconnection.c:169:9: error: format not a string literal, argument types not checked [-Werror=format-nonliteral]
         nscm);
         ^~~~
gunixconnection.c:203:20: error: format not a string literal, argument types not checked [-Werror=format-nonliteral]
                    nfd);
                    ^~~
gunixconnection.c: In function ‘g_unix_connection_receive_credentials’:
gunixconnection.c:566:24: error: format not a string literal, argument types not checked [-Werror=format-nonliteral]
                        nscm);
                        ^~~~