x11/libxcvt: switch to DISTVERSION after a2c98c9879ac

$ portlint -C
FATAL: Makefile: PORTVERSION appears out-of-order.
FATAL: Makefile: order must be PORTNAME/PORTVERSION/DISTVERSIONPREFIX/DISTVERSION/DISTVERSIONSUFFIX/PORTREVISION/PORTEPOCH/CATEGORIES/MASTER_SITES/MASTER_SITE_SUBDIR/PROJECTHOST/PKGNAMEPREFIX/PKGNAMESUFFIX/DISTNAME/EXTRACT_SUFX/DISTFILES(_\w+)?/DIST_SUBDIR/EXTRACT_ONLY.