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
 

docs/reference/gobject/gobject-sections.txt

@@ -398,7 +398,6 @@ G_TYPE_SOURCE
 G_TYPE_POLLFD
 G_TYPE_THREAD
 G_TYPE_OPTION_GROUP
-GStrv
 
 <SUBSECTION Standard>
 G_TYPE_IS_BOXED