Since commit v3.11.0-rc1~467^2 (VS,Xcode: Add CMakeLists.txt sources
without mutating targets, 2017-10-18) we do not add `CMakeLists.txt` to
target sources but instead generate references to them directly. This
accidentally dropped generation of the `.vcxproj.filters` entry for a
source group in which `CMakeLists.txt` is the only member.
Fixes: #18795
Brad King