qobject: Fix compiler warning (missing gnu_printf format attribute)

qobject: Fix compiler warning (missing gnu_printf format attribute)

gcc 4.8.2 reports this warning when extra warnings are enabled (-Wextra):

  CC    qobject/qerror.o
qobject/qerror.c: In function ‘qerror_from_info’:
qobject/qerror.c:53:5: error:
 function might be possible candidate for ‘gnu_printf’ format attribute [-Werror=suggest-attribute=format]
     qerr->err_msg = g_strdup_vprintf(fmt, *va);
     ^

Signed-off-by: Stefan Weil <sw@weilnetz.de>
Signed-off-by: Michael Tokarev <mjt@tls.msk.ru>