[NFC] JavaScriptEventLoop: Fix warnings for undeclared SPI usage

Author: kateinoigakukun

Sources/JavaScriptEventLoop/JavaScriptEventLoop+ExecutorFactory.swift

@@ -4,7 +4,10 @@
 // See: https://github.com/swiftlang/swift/pull/80266
 // See: https://forums.swift.org/t/pitch-2-custom-main-and-global-executors/78437
 
-@_spi(ExperimentalCustomExecutors) import _Concurrency
+#if compiler(>=6.3)
+@_spi(ExperimentalCustomExecutors)
+#endif
+import _Concurrency
 import _CJavaScriptKit
 
 #if compiler(>=6.3)

Sources/JavaScriptEventLoop/JavaScriptEventLoop.swift

@@ -1,5 +1,8 @@
 import JavaScriptKit
-@_spi(ExperimentalCustomExecutors) import _Concurrency
+#if compiler(>=6.3)
+@_spi(ExperimentalCustomExecutors)
+#endif
+import _Concurrency
 import _CJavaScriptEventLoop
 import _CJavaScriptKit