mosquitto: fix bug with firefox wss http/2

Firefox has a bug preventing websockets being created with
a server that uses http/2. Building libwebsockets without
http/2 support fixes this issue for mosquitto.
See eclipse-mosquitto/mosquitto#1211 (comment)

pkgs/servers/mqtt/mosquitto/default.nix

@@ -17,9 +17,14 @@
 
 let
   # Mosquitto needs external poll enabled in libwebsockets.
-  libwebsockets' = libwebsockets.override {
+  libwebsockets' = (libwebsockets.override {
     withExternalPoll = true;
-  };
+  }).overrideAttrs (old: {
+    # Avoid bug in firefox preventing websockets being created over http/2 connections
+    # https://github.com/eclipse/mosquitto/issues/1211#issuecomment-958137569
+    cmakeFlags = old.cmakeFlags ++ [ "-DLWS_WITH_HTTP2=OFF" ];
+  });
+
 in
 stdenv.mkDerivation rec {
   pname = "mosquitto";