Annotate API introduced for 2.30 with GLIB_AVAILABLE_IN_2_30

I didn't do this comprehensively, since there's a lot of it, mainly
due to the GDBus object manager stuff, but anyone trying to use
that would fail fast due to lack of the gdbus code generator.

My main goal was to get API additions to existing classes like
g_data_input_stream_read_line_utf8(), as well as the lower level new
API like glib-unix.h.

https://bugzilla.gnome.org/show_bug.cgi?id=676816

glib/gunicode.h

@@ -628,6 +628,7 @@ gchar*   g_utf8_find_prev_char    (const gchar *str,
 glong    g_utf8_strlen            (const gchar *p,
                                    gssize       max) G_GNUC_PURE;
 
+GLIB_AVAILABLE_IN_2_30
 gchar   *g_utf8_substring         (const gchar *str,
                                    glong        start_pos,
                                    glong        end_pos) G_GNUC_MALLOC;