Ninja: Fix CMP0058 on MinGW
For CMP0058 we identify dependencies that have no rules to generate them by collecting a set of all dependencies and a set of all files CMake knows are generated by something, and then computing a set difference. Fix construction of these sets to use the same form for each path. This is after ConvertToNinjaPath but before EncodePath. Without this, the slash conversion done by EncodePath for the GNU compiler on Windows results in a mismatch, triggering an incorrect CMP0058 warning.