Fixing compilation warnings

Change-Id: I3a1a361e6e8d40d2ac505b3c8d065c00224f1c2f
diff --git a/lib/arm_profiler/include/arm_profiler.hpp b/lib/arm_profiler/include/arm_profiler.hpp
index f5206fe..36adb8d 100644
--- a/lib/arm_profiler/include/arm_profiler.hpp
+++ b/lib/arm_profiler/include/arm_profiler.hpp
@@ -38,7 +38,7 @@
     std::unique_ptr<int32_t[]> start_ticks_;
     std::unique_ptr<int32_t[]> end_ticks_;
 
-    int num_events_ = 0;
+    size_t num_events_;
 
     TF_LITE_REMOVE_VIRTUAL_DELETE;
 };