libxcb: fix build with Buildroot compiled xsltproc

Make sure that libxcb depends on host-xsltproc, since xsltproc is
required for the compilation of libxcb.

Moreover, remove the XSLTPROC=/usr/bin/xsltproc variable definition,
which was assuming that XSLTPROC was installed on the host. xsltproc
should just be part of the PATH.

Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>