Provide decl for pango_font_metrics_new(), not declared in a public header
any more.

Index: pango.override
--- pango.override.orig
+++ pango.override
@@ -27,6 +27,8 @@ headers
 #include <pygobject.h>
 #include <pango/pango.h>
 
+PangoFontMetrics *pango_font_metrics_new(void);
+
 typedef struct {
     PyObject *func, *data;
 } PyGtkCustomNotify;