fix(dialyzer): fix dialyzer warnings

Adding hocon to emqx solves the issue, since other offending apps
depend on emqx.  The warnings were that it was not able to find info
on the `hocon_schema` behaviour.

Example:
```
Callback info about the hocon_schema behaviour is not available
```

apps/emqx/src/emqx.app.src

@@ -5,7 +5,19 @@
   {vsn, "5.0.0"}, % strict semver, bump manually!
   {modules, []},
   {registered, []},
-  {applications, [kernel,stdlib,gproc,gen_rpc,mria,esockd,cowboy,sasl,os_mon,jiffy,lc]},
+  {applications, [ kernel
+                 , stdlib
+                 , gproc
+                 , gen_rpc
+                 , mria
+                 , esockd
+                 , cowboy
+                 , sasl
+                 , os_mon
+                 , jiffy
+                 , lc
+                 , hocon
+                 ]},
   {mod, {emqx_app,[]}},
   {env, []},
   {licenses, ["Apache-2.0"]},