Fix static build on Windows (#15989)
Summary:
Tested locally. It could be now be started by running `set EXTRA_CAFFE2_CMAKE_FLAGS= -DTORCH_STATIC=1` before build. If we want to make sure it works, then maybe we should add it into CI.
Pull Request resolved: https://github.com/pytorch/pytorch/pull/15989
Differential Revision: D13649935
Pulled By: soumith
fbshipit-source-id: 956945ed572819d8cf0bc9bd48df3ea9bc6f4a8a
