currentcallview: fix GTK+ API incompatibility

gtk_css_provider_load_from_resource() was only added in GTK+ 3.16:
https://developer.gnome.org/gtk3/stable/GtkCssProvider.html#gtk-css-provider-load-from-resource

Debian 8 does not ship this version, so the client build currently
fails on this and other systems with < 3.16. We switch to using
load_from_data() version

note: you may have to clean your CMake build dir and re-run cmake
after this change, since our current cmake config doesn't work well
when gresource files are removed

Change-Id: I2a836a97cdea8b2d17bbb46f700a98d6eb153310
Reviewed-by: Guillaume Roguez <guillaume.roguez@savoirfairelinux.com>
3 files changed