message: elle.log: help GCC accepting the exhaustivity of a switch
diff: # include <syslog.h>
  #endif
  +#include <elle/assert.hh>
  #include <elle/log/SysLogger.hh>
  #include <elle/printf.hh>
  @@ -63,6 +64,7 @@ namespace elle
  case Logger::Type::warning:
  return LOG_WARNING;
  }
  + unreachable();
  }();
  auto msg = elle::sprintf("[%s] ", component);
  for (auto const& tag: tags)
  