Add API guard directives for native build

Author: kateinoigakukun

Sources/JavaScriptEventLoopTestSupport/JavaScriptEventLoopTestSupport.swift

@@ -19,7 +19,13 @@ import JavaScriptEventLoop
 
 // This module just expose 'JavaScriptEventLoop.installGlobalExecutor' to C ABI
 // See _CJavaScriptEventLoopTestSupport.c for why this is needed
+
+#if compiler(>=5.5)
+
+@available(macOS 10.15, iOS 13.0, watchOS 6.0, tvOS 13.0, *)
 @_cdecl("swift_javascriptkit_activate_js_executor_impl")
 func swift_javascriptkit_activate_js_executor_impl() {
     JavaScriptEventLoop.installGlobalExecutor()
 }
+
+#endif