fix for cad/gtkwave on macOS

[Yuri Honegger <yuri.honegger%icloud.com@localhost>]

Hi everyone,
Recently I’ve noticed that cad/gtkwave doesn’t build on macOS due to a missing dependency. The following fixes it.
I haven’t worked on pkgsrc before so I thought I’ll run it by the experts before I commit it.
- yuri

diff --git a/cad/gtkwave/Makefile b/cad/gtkwave/Makefile
index 36cdafe95e2b..ff8b4de22722 100644
--- a/cad/gtkwave/Makefile
+++ b/cad/gtkwave/Makefile
@@ -38,6 +38,10 @@ CONFIGURE_ARGS+=     --with-gconf
 .include "../../devel/gconfmm/buildlink3.mk"
 .endif

+.if ${OPSYS} == "Darwin"
+.include "../../x11/gtk-mac-integration/buildlink3.mk"
+.endif
+
 .include "../../archivers/bzip2/buildlink3.mk"
 .include "../../archivers/xz/buildlink3.mk"
 .include "../../devel/zlib/buildlink3.mk"