darwin-user: Use GCC_FMT_ATTR (format checking)

The redundant forward declaration of qerror in machload.c
is removed because it should be taken from qemu.h.

Please note that this patch is untested because
I have no matching environment to compile it.

Cc: Blue Swirl <blauwirbel@gmail.com>
Signed-off-by: Stefan Weil <weil@mail.berlios.de>
Signed-off-by: Blue Swirl <blauwirbel@gmail.com>
