Bug#1123915: libxdmcp: Build-Depends: w3m is unnecessary

[Jeremy Bícha <jbicha@debian.org>]

Actually, aclocal.m4 claims that either lynx, links, or w3m is needed
for the xmlto part of the build and simply removing w3m from
Build-Depends caused the package to fail to build. I have now used
Build-Depends: lynx since that's already installed for loong64
unstable (and everywhere else). So libxdmcp has now successfully been
built for loong64.

See https://bugs.debian.org/825146 which suggests that we might want
to add an Architecture: all libxdmcp-doc package to avoid this
bootstrapping problem.

Thank you,
Jeremy Bícha