FindBoost: Fix detection with version suffixes on Gentoo

The Gentoo case added by commit 1673923c30 (FindBoost: Add support for
Boost 1.67 with Python version suffixes, 2018-03-18, v3.11.0~3^2) left
out the `.` version component separator and instead duplicated the RPM
case.  Add the missing `.` now.

Fixes: #18743
3 files changed