tests: Skip "withIncludeFile" for PCH on Xcode backend

Xcode always fails here because this makes the PCH not be the first
included header, causing Xcode to ignore it completely. There is no
way around this.

test cases/common/13 pch/meson.build

@@ -13,7 +13,11 @@ subdir('cpp')
 subdir('generated')
 subdir('userDefined')
 subdir('withIncludeDirectories')
-subdir('withIncludeFile')
+if meson.backend() == 'xcode'
+  warning('Xcode backend does not support forced includes. Skipping "withIncludeFile" which requires this.')
+else
+  subdir('withIncludeFile')
+endif
 
 if meson.backend() == 'xcode'
   warning('Xcode backend only supports one precompiled header per target. Skipping "mixed" which has various precompiled headers.')