gnutls: Use ca-certificates as default trust store file
Since version 2.58 the glib-networking TLS database relies on GnuTLS's system
trust store, so not enabling it leads to TLS errors in applications depending on
glib-networking. The raised runtime warning is:
process:500): GLib-Net-WARNING **: 09:14:09.321: Failed to load TLS database: Failed to load system trust store: GnuTLS was not configured with a system trust
(app:490): ... TLS Error: TLS certificate has unknown CA.
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>
