[kcms/touchpad] fix X11 build
4a3f69dd broke my build with:
CMake Error at kcms/touchpad/CMakeLists.txt:62 (add_library):
Target "kded_touchpad" links to target "PkgConfig::XORGSERVER" but the
target was not found. Perhaps a find_package() call is missing for an
IMPORTED target, or an ALIAS target is missing?
I'm not sure this is the correct fix for all systems though.