wireless-security: fix failed assertion in default_filter_privkey()

The @out_encrypted argument of nm_utils_file_is_private_key() must be
initialized to FALSE. When using libnm-glib, keep the value set to
TRUE since encryption detection is not supported for all file types.

This fixes the following:

libnm-CRITICAL **: nm_utils_file_is_private_key: assertion 'out_encrypted == NULL || *out_encrypted == FALSE' failed

https://bugzilla.gnome.org/show_bug.cgi?id=763578
(cherry picked from commit 8e60431a)