package/libdvdread: fix static compile by adding libdvdcss as dependency

package/libdvdread: fix static compile by adding libdvdcss as dependency

If libdvdcss support was not found during configure libdvdread tries
to open libdvdcss.so nonetheless:
https://code.videolan.org/videolan/libdvdread/blob/master/src/dvd_input.c#L275

This will obviously fail in a static-only configuration. To fix the
problem add libdvdcss as hard dependency to this package.

Fixes
http://autobuild.buildroot.net/results/415/41534b70a66c6297652f0944f9ecc7e780b26c45/

Signed-off-by: Bernd Kuhls <bernd.kuhls@t-online.de>
[Thomas: replace += by =.]
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>