Add PORTROACH annotation.

This fixes portroach picking up a newer, incompatible version (which is
offered via unison/2.5x).

OK sthen@

net/unison/2.4x/Makefile

@@ -1,4 +1,4 @@
-# $OpenBSD: Makefile,v 1.1 2018/04/26 21:34:23 bket Exp $
+# $OpenBSD: Makefile,v 1.2 2018/05/02 08:08:11 bket Exp $
 
 VER=		2.48.15
 REV=		v3
@@ -6,6 +6,8 @@ REVISION=	0
 GH_TAGNAME=	v${VER}${REV}
 DISTNAME=	${GH_PROJECT}-${VER}
 
+PORTROACH=	limit:^2\.4
+
 WRKDIST=	${WRKDIR}/${PKGNAME}${REV}/src
 
 .include <bsd.port.mk>