Incorrect hard requirement for libxcvt

It's currently not possible to build things without having libxcvt installed, despite the fact that only the Xorg DDX requires it.

This limitation seems to have been a mistake, as libxcvt was added to both SDK_REQUIRED_MODULES, and has an explicit check in the Xorg code path. The fix is to simply remove it from SDK_REQUIRED_MODULES.

The meson file looks correct, so this should only affect the stable branch.

Issue discovered when building Xvnc with a newer Xorg. We had to add libxcvt to the build system, despite it never being used.