x11/slim: fix build on GCC architectures

Use new GCC:
/usr/local/poudriere/ports/default/x11/slim/work/slim-1.3.6/Ck.cpp: In member function 'const char* Ck::Session::get_x11_device(const std::string&)':
/usr/local/poudriere/ports/default/x11/slim/work/slim-1.3.6/Ck.cpp:94: error: 'snprintf' is not a member of 'std'

Approved by: mentors (implicit approval)