Renamed `flags` to `libprocess_flags` to fix Windows build.
Renamed `flags` to `libprocess_flags` to fix Windows build.

On Windows, the use of pre-compiled headers causes problems when reusing
the identifier `flags`. This commit renames it to `libprocess_flags` to
avoid conflicts.

Review: https://reviews.apache.org/r/59691/

asamerh4/mesos,asamerh4/mesos,jpeach/mesos,verizonlabs/mesos,abudnik/mesos,asamerh4/mesos,abudnik/mesos,zmalik/mesos,gsantovena/mesos,zmalik/mesos,asamerh4/mesos,abudnik/mesos,gsantovena/mesos,zmalik/mesos,kaysoky/mesos,dforsyth/mesos,chhsia0/mesos,dforsyth/mesos,kaysoky/mesos,reneploetz/mesos,asamerh4/mesos,chhsia0/mesos,verizonlabs/mesos,chhsia0/mesos,kaysoky/mesos,dforsyth/mesos,Gilbert88/mesos,gsantovena/mesos,chhsia0/mesos,zmalik/mesos,reneploetz/mesos,chhsia0/mesos,dforsyth/mesos,reneploetz/mesos,kaysoky/mesos,verizonlabs/mesos,asamerh4/mesos,gsantovena/mesos,andschwa/mesos,dforsyth/mesos,abudnik/mesos,verizonlabs/mesos,gsantovena/mesos,neilconway/mesos,gsantovena/mesos,kaysoky/mesos,asamerh4/mesos,verizonlabs/mesos,jpeach/mesos,Gilbert88/mesos,verizonlabs/mesos,verizonlabs/mesos,abudnik/mesos,kaysoky/mesos,reneploetz/mesos,zmalik/mesos,jpeach/mesos,gsantovena/mesos,Gilbert88/mesos,neilconway/mesos,andschwa/mesos,andschwa/mesos,abudnik/mesos,neilconway/mesos,zmalik/mesos,chhsia0/mesos,jpeach/mesos,andschwa/mesos,Gilbert88/mesos,Gilbert88/mesos,Gilbert88/mesos,chhsia0/mesos,reneploetz/mesos,zmalik/mesos,andschwa/mesos,reneploetz/mesos,jpeach/mesos,kaysoky/mesos,neilconway/mesos,neilconway/mesos,neilconway/mesos,andschwa/mesos,dforsyth/mesos,Gilbert88/mesos,dforsyth/mesos,andschwa/mesos,reneploetz/mesos,jpeach/mesos,jpeach/mesos,abudnik/mesos,neilconway/mesos