GCC 8.0's -Wcast-function-type objects casting function pointers to ones
with incompatible prototypes. Sometimes we do that on purpose though.

Notably, the g_source_set_callback()'s func argument can point to functions
of various prototypes. Also, libnm-glib/nm-remote-connection is perhaps
just not worth reworking, that would just be a waste of time.

A cast to void(*)(void) avoids the GCC warning, let's use it.