Fix stub definition for USE=-png
https://github.com/quakeforge/quakeforge/issues/14
--- a/libs/image/png.c
+++ b/libs/image/png.c
@@ -320,3 +320,3 @@
 VISIBLE tex_t *
-LoadPNG (QFile *infile)
+LoadPNG (QFile *infile, int load)
 {