Fix missing Windows import ucrt

Author: MaxDesiatov

Sources/WASI/Poll.swift

@@ -8,6 +8,8 @@ import SystemPackage
     import Musl
 #elseif canImport(Android)
     import Android
+#elseif os(Windows)
+    import ucrt
 #else
     #error("Unsupported Platform")
 #endif