Add explicit dependency on NIOTLS (#1244)

Motivation:

gRPC imports NIOTLS but does not decare its dependency on it in
Package.swift. This may lead to some build issues.

Modifications:

- Add dependency on NIOTLS to GRPC

Result:

Fewer build problems, hopefully.

Package.swift

@@ -60,6 +60,7 @@ let package = Package(
         .product(name: "NIOPosix", package: "swift-nio"),
         .product(name: "NIOEmbedded", package: "swift-nio"),
         .product(name: "NIOFoundationCompat", package: "swift-nio"),
+        .product(name: "NIOTLS", package: "swift-nio"),
         .product(name: "NIOTransportServices", package: "swift-nio-transport-services"),
         .product(name: "NIOHTTP1", package: "swift-nio"),
         .product(name: "NIOHTTP2", package: "swift-nio-http2"),