d/control: Avoid libwacom-dev when not using xserver-xorg-input-wacom
The upstream build system uses the presence or absence of libwacom-dev to decide whether to build the Wacom plugin, which requires xorg-wacom.pc from xserver-xorg-input-wacom. That package is unavailable on s390x (and presumably Hurd and kFreeBSD), causing build failure.
Avoid this by dropping the build-dependency on libwacom-dev on architectures where xserver-xorg-input-wacom can't be installed, and moving it to Build-Conflicts on those architectures.
Closes: #977762
Edited by Simon McVittie