support Swift 5.1 on Linux (#610)

Tests/SwiftGRPCNIOTests/NIOServerWebTests.swift

@@ -14,6 +14,9 @@
  * limitations under the License.
  */
 import Foundation
+#if canImport(FoundationNetworking)
+import FoundationNetworking
+#endif
 import NIO
 @testable import SwiftGRPCNIO
 import XCTest