folly/.../FileUtilDetail.h: avoid -Wsign-compare warnings

folly/.../FileUtilDetail.h: avoid -Wsign-compare warnings

Summary:
* folly/detail/FileUtilDetail.h (wrapvFull): Change type of result
variable from ssize_t to size_t: this required changing r == -1 to
r == (size_t)-1.

Test Plan:
Run this and note there are fewer errors than before:
fbconfig --platform-all=gcc-4.9-glibc-2.20 -r folly && fbmake dbgo
Also run fbmake runtests, and confirm that the results are
the same with and without this change.

Reviewed By: lucian@fb.com

Subscribers: folly-diffs@

FB internal diff: D1772171

Tasks: 5941250

Signature: t1:1772171:1420743142:1f9e02006e2d77a0c9451bae7965fb28617d95d6