libc: Add missing MLINK for stravis(3)

MFC after:	1 week
Fixes:		8dfeba04eb ("Update to a June 8th snapshot of (un)vis form NetBSD.")
Reviewed by:	ziaee
Differential Revision:	https://reviews.freebsd.org/D56260

lib/libc/gen/Makefile.inc

@@ -562,6 +562,7 @@ MLINKS+=unvis.3 strunvis.3 \
 	unvis.3 strunvisx.3
 MLINKS+=vis.3 nvis.3 \
 	vis.3 snvis.3 \
+	vis.3 stravis.3 \
 	vis.3 strenvisx.3 \
 	vis.3 strnunvis.3 \
 	vis.3 strnunvisx.3 \