Fix compilation on BSD platforms

I could only test this with FreeBSD and OSX
(on the Node.js CI).
I don't know if the fix is correct for other BSD platforms.

Review-Url: https://codereview.chromium.org/2251603004
Cr-Commit-Position: refs/heads/master@{#38905}

src/libsampler/sampler.cc

@@ -193,7 +193,7 @@ void* ThreadKey(pthread_t thread_id) {
 
 // Returns hash value for hash map.
 uint32_t ThreadHash(pthread_t thread_id) {
-#if V8_OS_MACOSX
+#if V8_OS_BSD
   return static_cast<uint32_t>(reinterpret_cast<intptr_t>(thread_id));
 #else
   return static_cast<uint32_t>(thread_id);