Mark libxcvt as required

It's used unconditionally later on

Make sure it fails at the appropriate time when not found

CMakeLists.txt

@@ -245,7 +245,7 @@ else()
     set(HAVE_GBM_BO_GET_FD_FOR_PLANE 0)
 endif()
 
-pkg_check_modules(Libxcvt IMPORTED_TARGET libxcvt>=0.1.1)
+pkg_check_modules(Libxcvt IMPORTED_TARGET libxcvt>=0.1.1 REQUIRED)
 add_feature_info(Libxcvt Libxcvt_FOUND "Required for generating modes in the drm backend")
 
 find_package(X11)