fix: Guard AppKit against macOS (#15)

Author: jbmorley

Sources/SelectableCollectionView/Utilities/NavigationDirection.swift

@@ -18,17 +18,21 @@
 // OUT OF OR IN CONNECTION WITH THE SOFTWARE OR THE USE OR OTHER DEALINGS IN THE
 // SOFTWARE.
 
-#if os(macOS)
 
+import Foundation
+
+#if os(macOS)
 import AppKit
 import Carbon
+#endif
 
 enum NavigationDirection {
     case up
     case down
     case left
     case right
 
+#if os(macOS)
     init?(_ keyCode: UInt16) {
         switch Int(keyCode) {
         case kVK_LeftArrow:
@@ -43,7 +47,6 @@ enum NavigationDirection {
             return nil
         }
     }
+#endif
 
 }
-
-#endif