gnome3: correctly detect when no GNOME screenlock exists

* gnome3/pinentry-gnome3.c (pe_gnome_screen_locked): skip
G_IO_ERROR_IS_DIRECTORY responses, not G_IO_ERROR_TIMED_OUT responses
from dbus.

After the previous commit, testing demonstrates that the normal
response when no screenlock is running is G_IO_ERROR_IS_DIRECTORY, not
G_IO_ERROR_TIMED_OUT.

• Debian-bug-id: #927105
• Signed-off-by: Daniel Kahn Gillmor <dkg@fifthhorseman.net>