Allow targeting/building with old SDKs when FBRULnetworkServiceTypeCallSignalling is not available.

SocketRocket/Internal/Utilities/SRURLUtilities.m

@@ -72,11 +72,13 @@ extern NSString *_Nullable SRStreamNetworkServiceTypeFromURLRequest(NSURLRequest
         case NSURLNetworkServiceTypeVoice:
             networkServiceType = NSStreamNetworkServiceTypeVoice;
             break;
+#if (__MAC_OS_X_VERSION_MAX_ALLOWED >= 101200 || __IPHONE_OS_VERSION_MAX_ALLOWED >= 100000 || __TV_OS_VERSION_MAX_ALLOWED >= 100000 || __WATCH_OS_VERSION_MAX_ALLOWED >= 30000)
         case NSURLNetworkServiceTypeCallSignaling: {
             if (@available(iOS 10.0, tvOS 10.0, macOS 10.12, *)) {
                 networkServiceType = NSStreamNetworkServiceTypeCallSignaling;
             }
         } break;
+#endif
     }
     return networkServiceType;
 }