Windows: Fixed warnings in `getcwd.hpp`.
Windows: Fixed warnings in `getcwd.hpp`.

This removes an implicit cast from unsigned to signed int.

Review: https://reviews.apache.org/r/52210/

jpeach/mesos,dforsyth/mesos,kaysoky/mesos,neilconway/mesos,Aman-Jain-14/customizedMesos,kaysoky/mesos,kaysoky/mesos,asamerh4/mesos,yuquanshan/customizedMesos,yuquanshan/customizedMesos,jpeach/mesos,zmalik/mesos,neilconway/mesos,jpeach/mesos,craimbert/mesos,andschwa/mesos,jpeach/mesos,zmalik/mesos,andschwa/mesos,Aman-Jain-14/customizedMesos,reneploetz/mesos,verizonlabs/mesos,kaysoky/mesos,craimbert/mesos,yuquanshan/customizedMesos,neilconway/mesos,Gilbert88/mesos,asamerh4/mesos,reneploetz/mesos,gsantovena/mesos,kaysoky/mesos,kaysoky/mesos,reneploetz/mesos,neilconway/mesos,chhsia0/mesos,yuquanshan/customizedMesos,dforsyth/mesos,chhsia0/mesos,craimbert/mesos,chhsia0/mesos,asamerh4/mesos,jpeach/mesos,abudnik/mesos,kaysoky/mesos,zmalik/mesos,Gilbert88/mesos,dforsyth/mesos,andschwa/mesos,asamerh4/mesos,abudnik/mesos,jpeach/mesos,Aman-Jain-14/customizedMesos,yuquanshan/customizedMesos,Aman-Jain-14/customizedMesos,abudnik/mesos,craimbert/mesos,zmalik/mesos,verizonlabs/mesos,Gilbert88/mesos,asamerh4/mesos,abudnik/mesos,abudnik/mesos,gsantovena/mesos,reneploetz/mesos,Gilbert88/mesos,reneploetz/mesos,yuquanshan/customizedMesos,dforsyth/mesos,neilconway/mesos,zmalik/mesos,neilconway/mesos,chhsia0/mesos,verizonlabs/mesos,gsantovena/mesos,chhsia0/mesos,verizonlabs/mesos,dforsyth/mesos,zmalik/mesos,gsantovena/mesos,gsantovena/mesos,andschwa/mesos,verizonlabs/mesos,andschwa/mesos,abudnik/mesos,dforsyth/mesos,gsantovena/mesos,Gilbert88/mesos,Aman-Jain-14/customizedMesos,jpeach/mesos,yuquanshan/customizedMesos,chhsia0/mesos,andschwa/mesos,dforsyth/mesos,chhsia0/mesos,asamerh4/mesos,reneploetz/mesos,Aman-Jain-14/customizedMesos,verizonlabs/mesos,zmalik/mesos,Aman-Jain-14/customizedMesos,craimbert/mesos,abudnik/mesos,craimbert/mesos,andschwa/mesos,gsantovena/mesos,reneploetz/mesos,Gilbert88/mesos,asamerh4/mesos,neilconway/mesos,Gilbert88/mesos,verizonlabs/mesos,craimbert/mesos