Fix error reports on missing include paths

In some cases (nested includes?) paths end up in such a way that joining
them breaks up and hard-crashes rebar3. This patch specifically handles
this scenario to fix things by avoiding passing empty lists to
filename:join.

src/rebar_erlc_compiler.erl

@@ -688,7 +688,7 @@ warn_and_find_path(File, Dir) ->
         true ->
             [SrcHeader];
         false ->
-            IncludeDir = filename:join(filename:join(rebar_utils:droplast(filename:split(Dir))), "include"),
+            IncludeDir = filename:join(rebar_utils:droplast(filename:split(Dir))++["include"]),
             IncludeHeader = filename:join(IncludeDir, File),
             case filelib:is_regular(IncludeHeader) of
                 true ->