make the case not ENABLE_QUICKSTART_LIBPNG work again

desktop/unx/source/splashx.c

@@ -781,6 +781,12 @@ struct splash* splash_create(rtl_uString* pAppPath, int argc, char** argv)
 
 #else /* not ENABLE_QUICKSTART_LIBPNG */
 
+#include <rtl/ustrbuf.h>
+
+struct splash
+{
+};
+
 /* Stubs that will never be called in this case */
 void splash_draw_progress( struct splash* splash, int progress )
 {