NIOPosix: add missing import for Windows (#2188)

Add an import of `NIOCore` on Windows which mirrors the other platforms.
This greatly reduces the noise in the error list.

Co-authored-by: Cory Benfield <lukasa@apple.com>

Sources/NIOPosix/BSDSocketAPIWindows.swift

@@ -12,6 +12,8 @@
 //
 //===----------------------------------------------------------------------===//
 
+import NIOCore
+
 #if os(Windows)
 import ucrt