These functions are only declared in the non-installed, private header
file.  Add declarations for now, so that things keep building.

This looks like it fixes a pointer trucation issue.

Submitted upstream: <https://rt.cpan.org/Ticket/Display.html?id=145548>

diff -ur Goo-Canvas-0.06.orig/goocanvas-perl.h Goo-Canvas-0.06/goocanvas-perl.h
--- Goo-Canvas-0.06.orig/goocanvas-perl.h	2007-09-29 02:59:38.000000000 +0200
+++ Goo-Canvas-0.06/goocanvas-perl.h	2022-12-09 21:09:33.927299572 +0100
@@ -9,6 +9,10 @@
 #include "cairo-perl.h"
 #include "goocanvas.h"
 
+/* From goocanvasprivate.h.  */
+cairo_pattern_t* goo_canvas_cairo_pattern_from_pixbuf (GdkPixbuf *pixbuf);
+cairo_surface_t* goo_canvas_cairo_surface_from_pixbuf (GdkPixbuf *pixbuf);
+
 /* FIXME: How to avoid compile error in autogen.h */
 #define GooCairoPattern cairo_pattern_t
 #define GooCairoMatrix cairo_matrix_t