xorg-server: add dependency on dri2proto when systemd is built

xorg-server: add dependency on dri2proto when systemd is built

When systemd is selected, xorg-server must have config-udev-kms
enabled in order to build correctly, but it won't be the case if
BR2_PACKAGE_XPROTO_DRI2PROTO is not part of the build.

Add BR2_PACKAGE_XPROTO_DRI2PROTO as a dependency of xorg-xserver
only when systemd is part of the build.

Fixes bug #9486.

Signed-off-by: Fabien Parent <fparent@baylibre.com>
Tested-by: Bernd Kuhls <bernd.kuhls@t-online.de>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>