tests: fix unsigned/signed comparison in misc-free-perf.cc

thread_allocator::next_core is compared against sched::cpus.size().
GCC 4.9 doesn't like the fact that the former is signed and the latter
is unsigned.

Reviewed-by: Tomasz Grabiec <tgrabiec@gmail.com>
Signed-off-by: Paweł Dziepak <pdziepak@quarnos.org>
Signed-off-by: Pekka Enberg <penberg@cloudius-systems.com>

tests/misc-free-perf.cc

@@ -28,7 +28,7 @@ class thread_allocator
 {
 private:
     std::vector<sched::thread*> threads;
-    int next_core {};
+    unsigned next_core {};
 public:
     template<typename Func>
     void add(Func func)
@@ -145,4 +145,4 @@ int main(int argc, char const *argv[])
 {
     test_across_core_alloc_and_free(std::bind(malloc, 1024), free);
     return 0;
-}
\ No newline at end of file
+}