Skip functions from JNAeration based on regexp

The GNOME C coding style document
(https://help.gnome.org/users/programming-guidelines/stable/c-coding-style.html.en)
explicitly states that "Underscore-prefixed functions are never
exported". GLib uses the GNOME coding style guidelines, therefore
any function with a name that begins with an underscore should be
skipped to prevent BridJ warnings. See the Private API part in the
linked document for the original details.