CMake: Don't run sdk / xcode version checks on non-Apple platforms

It was an oversight.

Amends a29bff3d80

Pick-to: 6.6
Change-Id: I696d7a2650c336dc93df01245065bcbb61ca1d6b
Reviewed-by: Alexey Edelev <alexey.edelev@qt.io>

cmake/QtPublicAppleHelpers.cmake

@@ -781,6 +781,10 @@ endfunction()
 #
 # The qmake equivalent for user projects is in mkspecs/features/mac/default_post.prf.
 function(_qt_internal_check_apple_sdk_and_xcode_versions)
+    if(NOT APPLE)
+        return()
+    endif()
+
     if(QT_NO_APPLE_SDK_AND_XCODE_CHECK)
         return()
     endif()