don't compile dnnlowp.cc in avx2 option (#15147)
Summary:
Pull Request resolved: https://github.com/pytorch/pytorch/pull/15147
Forgot to take out dnnlowp.cc from avx2 list in a previous diff.
Reviewed By: dskhudia
Differential Revision: D13440686
fbshipit-source-id: 9ada98b6e885c7d5f22c91a735ff60304480b4cb
