Move GStrv typedef to glib.h instead of gobject.h

GStrv was historically only needed for the boxed G_TYPE_STRV,
but it is now useful for g_auto(GStrv) as well. This is not
an ABI change.

https://bugzilla.gnome.org/show_bug.cgi?id=755355

docs/reference/glib/glib-sections.txt

@@ -1453,6 +1453,9 @@ g_strfreev
 g_strconcat
 g_strjoin
 g_strjoinv
+
+<SUBSECTION>
+GStrv
 g_strv_length
 g_strv_contains