web-source: fix type mismatch on unused parameter

src/gclue-web-source.c:181:35: warning: passing argument 2 of ‘on_network_changed’ makes integer from pointer without a cast [-Wint-conversion]
         on_network_changed (NULL, NULL, user_data);
                                   ^~~~
src/freedesktop/geoclue/geoclue/src/gclue-web-source.c:135:38: note: expected ‘gboolean’ {aka ‘int’} but argument is of type ‘void *’
                     gboolean         available G_GNUC_UNUSED,
3 jobs for glib-2.44 in 6 minutes and 43 seconds (queued for 3 seconds)
latest
Status Job ID Name Coverage
  Test
passed #40759
ubuntu-16.04

00:06:43

passed #40760
ubuntu-17.10

00:06:19

passed #40761
ubuntu-18.04

00:06:04