network: Fix 802.1x TLS file chooser filters

If on the Security tab in the connection editor one selected 802.1x with TLS authentication, the 3 file chooser dialogs there didn't let one select any file. This was because allowed extensions were passed to gtk_file_filter_add_suffix () with the leading dot but that function expects them without the leading dot. Adjusted the code to skip over the dot.

Fixes #2022 (closed).