Exclude Xcode project files from whitespace style checks

These files contain TABs and didn't pass the check and, of course, we're
not interested in the style in which they're written anyhow.

.github/workflows/code_checks.yml

@@ -47,6 +47,7 @@ jobs:
                 ':!descrip.mms' \
                 ':!install-sh' \
                 ':!**/*akefile*' \
+                ':!**/*.pbxproj' \
                 ':!**/*.sln' \
                 ':!**/*.vcproj' \
                 ':!**/*.xpm'