call Gtk3::init manually

call Gtk3::init manually

When using the module -init switch, the init method was called in
module import method, which means every time this was loaded, even if
just for a quick syntax+ check using `perl -I. -wc`, failing then if
no X11 connection was available.

Calling this manually after module load finished should have no
consequences, and if, that should be fixed there, without going back
to doing GUI init in module load/import steps - that's just ugly.

Signed-off-by: Thomas Lamprecht <t.lamprecht@proxmox.com>