qga/channel-posix.c: Include headers it needs

qga/channel-posix.c: Include headers it needs

Include:
 - <errno.h> for errno
 - <unistd.h> & <fcntl.h> for fcntl()
 - <stdlib.h> for exit()
 - "osdep.h" for qemu_open()

Some of those headers were probably being included by accident because
some other headers were including qemu-common.h, but those headers
should eventually stop including qemu-common.h.

Signed-off-by: Eduardo Habkost <ehabkost@redhat.com>
Signed-off-by: Igor Mammedov <imammedo@redhat.com>
Signed-off-by: Andreas Färber <afaerber@suse.de>