netsurf: Add dependency to libiconv

Pointed out via PR pkg/54137 by Jonathan Schleifer.

Bump PKGREVISION

Index: Makefile
===================================================================
RCS file: /cvsroot/pkgsrc/www/netsurf/Makefile,v
retrieving revision 1.73
diff -u -p -r1.73 Makefile
--- Makefile	13 Dec 2018 19:52:25 -0000	1.73
+++ Makefile	23 Apr 2019 22:22:47 -0000
@@ -2,7 +2,7 @@
 
 DISTNAME=	netsurf-all-3.8
 PKGNAME=	${DISTNAME:S/-all//}
-PKGREVISION=	3
+PKGREVISION=	4
 CATEGORIES=	www
 MASTER_SITES=	http://download.netsurf-browser.org/netsurf/releases/source-full/
 
@@ -38,6 +38,7 @@ BUILDLINK_TRANSFORM+=	rm:-Werror
 CFLAGS.NetBSD+=		-D_NETBSD_SOURCE	# needed for strcasestr(3) and strndup(3)
 
 .include "../../x11/gtk2/buildlink3.mk"
+.include "../../converters/libiconv/buildlink3.mk"
 .include "../../devel/gperf/buildlink3.mk"
 .include "../../devel/libglade/buildlink3.mk"
 .include "../../graphics/librsvg/buildlink3.mk"