cmd/dist: fix build
cmd/dist: fix build

The Unix and Plan 9 readfile call breset(b) but Windows was not,
leaving dregs in the buffer.

TBR=golang-dev
CC=golang-dev
https://codereview.appspot.com/7229069
abustany/go,abustany/go,abustany/go,abustany/go,abustany/go,abustany/go,abustany/go