folly/Format-inl.h: use the correct type for fwrite return value (easy)

folly/Format-inl.h: use the correct type for fwrite return value (easy)

Summary:
* folly/Format-inl.h (folly): fwrite returns size_t, not ssize_t.
Fixing this avoids the following diagnostic from gcc-4.9:
folly/Format-inl.h:299:11: error: comparison between signed and unsigned integer expressions [-Werror=sign-compare]

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

Reviewed By: philipp@fb.com

Subscribers: trunkagent, net-systems@, folly-diffs@

FB internal diff: D1770155

Tasks: 5941250

Signature: t1:1770155:1420668733:62593231d35f5874521dba575f23b68891b2a4f9