build: only add -Fsystem path if configured

The system framework paths are only configured for the Apple targets.  Add a
condition to avoid a search path to be added for other SDKs.

Author: compnerd

cmake/modules/AddSwift.cmake

@@ -1878,7 +1878,7 @@ function(add_swift_target_library name)
       endforeach()
 
       # Add PrivateFrameworks, rdar://28466433
-      if(SWIFTLIB_IS_SDK_OVERLAY)
+      if(sdk IN_LIST SWIFT_APPLE_PLATFORMS AND SWIFTLIB_IS_SDK_OVERLAY)
         set(swiftlib_swift_compile_private_frameworks_flag "-Fsystem" "${SWIFT_SDK_${sdk}_ARCH_${arch}_PATH}/System/Library/PrivateFrameworks/")
       endif()