ext_mod: Support library path installed from Debian (processone/ejabberd-contrib#363)

Author: badlop

src/ext_mod.erl

@@ -645,6 +645,8 @@ compile_options() ->
     ++ maybe_define_lager_macro()
     ++ [{i, filename:join(app_dir(App), "include")}
         || App <- [fast_xml, xmpp, p1_utils, ejabberd]]
+    ++ [{i, filename:join(app_dir(App), "include")}
+        || App <- [p1_xml, p1_xmpp]] % paths used in Debian packages
     ++ [{i, filename:join(mod_dir(Mod), "include")}
         || Mod <- installed()].