vala: fix documentation build problem

On gcc10, the build of the host-vala documentation causes a
segmentation fault of xsltproc, for some unknown reason. See:

  http://autobuild.buildroot.org/results/42fea25ac4fe33a56c02d90c361da1c494f79ba3/build-end.log



Therefore, this patch fixes the problem by:

 * Ensuring that host-libxslt is amongst the dependency of vala when
   documentation is enabled so that we build a well-known version of
   libxslt/xsltproc to build the documentation. If documentation is
   not enabled, then we force vala to *not* find xsltproc so that it
   doesn't try to use the xsltproc of the build machine distribution
   if available.

 * Disabling the build of the documentation for host-vala. Note that
   we need to pass ':' as the xsltproc binary name to teach the
   configure script that xsltproc is not available.

Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>
Signed-off-by: Peter Korsgaard <jacmet@sunsite.dk>