tools/libs/vchan: Don't run the headers check

tools/libs/vchan: Don't run the headers check

There was never a headers check previously, and CentOS 6 can't cope with the
anonymous union in struct libxenvchan.

  cc1: warnings being treated as errors
  ... tools/include/libxenvchan.h:75: error: declaration does not declare anything
  make[6]: *** [headers.chk] Error 1

Fixes: 8ab2429f12 ("tools: split libxenvchan into new tools/libs/vchan directory")
Signed-off-by: Andrew Cooper <andrew.cooper3@citrix.com>
Acked-by: Wei Liu <wl@xen.org>