For g_autolist and g_autoslist, the cleanup func was cast to
GDestroyNotify before being passed to g_(s)list_free_full. This cast
provokes GCC 8 to emit a warning if the return type is not void:

    …/gmacros.h:462:99: warning: cast between incompatible function types
    from … to 'void (*)(void *)' [-Wcast-function-type]

Cast to 'void (*)(void)' first, which suppresses the warning as
recommended by the GCC documentation. g_autoptr remains untouched.

Fixes #1382