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

folly/test/SpinLockTest.cpp

@@ -60,7 +60,7 @@ struct TryLockState {
 };
 
 template <typename LOCK>
-void trylockTestThread(TryLockState<LOCK>* state, int count) {
+void trylockTestThread(TryLockState<LOCK>* state, size_t count) {
   while (true) {
     asm("pause");
     SpinLockGuardImpl<LOCK> g(state->lock1);
@@ -111,7 +111,7 @@ void trylockTest() {
   int nthrs = sysconf(_SC_NPROCESSORS_ONLN) + 4;
   std::vector<std::thread> threads;
   TryLockState<LOCK> state;
-  int count = 100;
+  size_t count = 100;
   for (int i = 0; i < nthrs; ++i) {
     threads.push_back(std::thread(trylockTestThread<LOCK>, &state, count));
   }