BUILD/MINOR: server: fix compilation without SSL

The build without SSL was broken by the following backported commit:
  8e99b84dba7755e73563093a8fcedb25e70e7b94
  MEDIUM: server/backend: implement websocket protocol selection

Fix this by checking the ALPN only if USE_OPENSSL and
TLSEXT_TYPE_application_layer_protocol_negotiation are defined. Else
always use the server mux-proto.

No need to backport.
1 file changed