docker: depend on xlib_libX11 and not the X.org server

docker is a X client application, so it doesn't make sense to depend
on the X.org server. An inspection of docker Makefile and source code
shows that it only needs the libX11 library, so we replace the X.org
server dependency by a libX11 dependency.

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