x11/wcm: explicitly require wayfire to be installed

$ wcm
[src/file.cpp:441] Failed to open XML directory /usr/local/share/wayfire/metadata
[src/file.cpp:441] Failed to open XML directory /usr/local/share/wayfire/metadata/wf-shell
Process 91210 stopped

• thread #1, name = 'wcm', stop reason = signal SIGSEGV: invalid address (fault address: 0x8) frame #0: 0x0000000801164ea5 libwf-config.so.1`std::1::unique_ptr<wf::config::section_t::impl, std::1::default_delete<wf::config::section_t::impl> >::operator->(this=0x0000000000000008) const at memory:2587:19 2584 } 2585 _LIBCPP_INLINE_VISIBILITY 2586 pointer operator->() const _NOEXCEPT {

-> 2587 return __ptr_.first();

2588   }
2589   _LIBCPP_INLINE_VISIBILITY
2590   pointer get() const _NOEXCEPT {

(lldb) bt

• thread #1, name = 'wcm', stop reason = signal SIGSEGV: invalid address (fault address: 0x8)
  • frame #0: 0x0000000801164ea5 libwf-config.so.1`std::1::unique_ptr<wf::config::section_t::impl, std::1::default_delete<wf::config::section_t::impl> >::operator->(this=0x0000000000000008) const at memory:2587:19 frame #1: 0x00000008011647ec libwf-config.so.1`wf::config::section_t::get_option_or(this=0x0000000000000000, name="plugins") at section.cpp:37:9 frame #2: 0x00000008011648a7 libwf-config.so.1`wf::config::section_t::get_option(this=0x0000000000000000, name="plugins") at section.cpp:46:19 frame #3: 0x0000000000217453 wcm`init(wcm=0x000000080251b320) at main.cpp:221:29 frame #4: 0x00000000002172e2 wcm`main(argc=1, argv=0x00007fffffffeb70) at main.cpp:257:5 frame #5: 0x0000000000216750 wcm`_start(ap=<unavailable>, cleanup=<unavailable>) at crt1.c:76:7