protoc: only disable in watch os mode
protoc: only disable in watch os mode

Summary:
(see comment)
Closes https://github.com/caffe2/caffe2/pull/1157

Reviewed By: bddppq

Differential Revision: D5753813

Pulled By: Yangqing

fbshipit-source-id: 7a60f03bb37314161e42ac0405a4b168d2541f3f

davinwang/caffe2,sf-wind/caffe2,pietern/caffe2,davinwang/caffe2,sf-wind/caffe2,Yangqing/caffe2,xzturn/caffe2,Yangqing/caffe2,sf-wind/caffe2,davinwang/caffe2,caffe2/caffe2,sf-wind/caffe2,pietern/caffe2,pietern/caffe2,Yangqing/caffe2,xzturn/caffe2,davinwang/caffe2,Yangqing/caffe2,davinwang/caffe2,xzturn/caffe2,pietern/caffe2,pietern/caffe2,xzturn/caffe2,Yangqing/caffe2,sf-wind/caffe2,xzturn/caffe2