x11/wcm: unbreak build with libc++ < 16 after 275590e5fab0

In file included from ../src/wcm.cpp:1:
../src/wcm.hpp:69:52: error: implicit instantiation of undefined template 'std::array<Gtk::Separator, 7>'

std::array<Gtk::Separator, NUM_CATEGORIES - 1> separators;
                                               ^

/usr/include/c++/v1/__tuple:219:64: note: template is declared here
template <class _Tp, size_t _Size> struct _LIBCPP_TEMPLATE_VIS array;

^

PR: 274347
Reported by: Chad Jacob Milios