[gn build] minor tweaks to sync script

- only complain about duplicate 'sources' lines if they contain
  more than 1 elements each (before, 0 and 1 element lists were
  counted too). In practice, this only has an effect for
  clang/lib/Headers/BUILD.gn

- make the '# NOSORT' diag actually work. This too only affects
  clang/lib/Headers/BUILD.gn.

In aggregate, changes to clang/lib/Headers/BUILD.gn still can't
be auto-merged, but for a slighly better reason.