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.
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.