.gitignore: add Visual Studio CMakeSetting.json file

The CMakeSettings.json file is tool generated. Developers may track it
should they provide additional settings.

Signed-off-by: Philip Oakley <philipoakley@iee.email>

.gitignore

@@ -250,3 +250,4 @@ Release/
 /git.VC.db
 *.dSYM
 /contrib/buildsystems/out
+CMakeSettings.json