security/klee: fix build with GCC-based architectures

Add USES=compiler:c++14-lang to fix build on GCC architectures:
Make Error in lib/Basic/CMakeLists.txt:

Target "kleeBasic" requires the language dialect "CXX14" (with compiler
extensions), but CMake does not know the compile flags to use to enable it.

PR: 237623
Approved by: arrowd (maintainer), mat (mentor)
Differential Revision: https://reviews.freebsd.org/D20088