Add missing import of Network module

In #794, the new Swift
compiler feature `MemberImportVisibility` was adopted. However, CI for this
project does not test building the package with the nightly compiler on macOS,
so it was missed that the project no longer builds on macOS with the nightly
toolchain due to a missing import.

Author: tshortli

Sources/AsyncHTTPClient/ConnectionPool/HTTPConnectionPool+Factory.swift

@@ -22,6 +22,7 @@ import NIOSSL
 import NIOTLS
 
 #if canImport(Network)
+import Network
 import NIOTransportServices
 #endif