net/tigervnc: update 1.7.0 -> 1.7.1

PR: 216387
Submitted by: Koichiro IWAO <meta+ports@vmeta.jp> (maintainer)
Changes: https://github.com/TigerVNC/tigervnc/releases/tag/v1.7.1
MFH: 2017Q1
Security: CVE-2017-5581