gmain: Mark G_SOURCE_FUNC as available in 2.58
gmain: Mark G_SOURCE_FUNC as available in 2.58

This means that GLib will generate appropriate deprecation warnings if
`GLIB_VERSION_MAX_ALLOWED` has been set to target a GLib version older
than 2.58.

endlessm/glib,endlessm/glib,endlessm/glib,endlessm/glib,endlessm/glib