irc/quasssel: Patch CMakeLists.txt to fix build against SSL with Qt 5.6

PR: 213581
Submitted by: Christian Schwarz (me@cschwarz.com)
Submitted by: tcberner
Reviewed by: mat
Approved by: adamw (mentor, implicit)
Obtained from: https://github.com/quassel/quassel/commit/4768c9e99f99b581d4e32e797db91d0182391696
MFH: 2016Q4
Differential Revision: https://reviews.freebsd.org/D8294