Fix mac build breaks

Signed-off-by: Keith Mok <ek9852@gmail.com>
Change-Id: I4b2926342bbf8621f7b7f5695cf1526dd7281bef
diff --git a/src/armnnUtils/Threads.cpp b/src/armnnUtils/Threads.cpp
index 64a8481..e5cf276 100644
--- a/src/armnnUtils/Threads.cpp
+++ b/src/armnnUtils/Threads.cpp
@@ -13,6 +13,7 @@
 #include <common/include/WindowsWrapper.hpp>
 #elif defined(__APPLE__)
 #include "AvailabilityMacros.h"
+#include <pthread.h>
 #include <sys/syscall.h>
 #include <sys/time.h>
 #include <unistd.h>
@@ -36,7 +37,7 @@
     {
         return 0;
     }
-    return threadId;
+    return static_cast<int>(threadId);
 #endif
 }