ExampleSwiftLibrary: adjust imports for Windows

Guard the `Darwin` import to when the module is available rather than
assuming it to be the default. Add a case for Windows.

Author: compnerd

Sources/ExampleSwiftLibrary/MySwiftLibrary.swift

@@ -19,7 +19,9 @@
 
 #if os(Linux)
 import Glibc
-#else
+#elseif os(Windows)
+import CRT
+#elseif canImport(Darwin)
 import Darwin.C
 #endif