plugin clean proto output files failed.

[raincoldl]

rebar3 protobuf clean can not clean the dir, plz fix this problem in rebar3_gpb_compiler clean function.

The code part below can not return the right filename of those output files.
GeneratedRootFiles =
lists:usort(
[filename:rootname(get_target(ProtoFile, GpbOpts))
|| ProtoFile <- ProtoFiles]),