folly/test/SpinLockTest.cpp: avoid -Wsign-compare error (trivial)

folly/test/SpinLockTest.cpp: avoid -Wsign-compare error (trivial)

Summary:
* folly/test/SpinLockTest.cpp (trylockTestThread):
Change parameter type from int to size_t, to fix these:
folly/test/SpinLockTest.cpp:67:25: error: comparison between signed and unsigned integer expressions [-Werror=sign-compare]
folly/test/SpinLockTest.cpp:82:60: 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: net-systems@, folly-diffs@

FB internal diff: D1770564

Tasks: 5941250

Signature: t1:1770564:1420674373:fa3020398e2df66590eb71f798419b6f555d07c4