Add explicit return in the default case (#1250)

Summary:
Fixes cases where control would reach the end of the setThreadName function without an explicit return
Pull Request resolved: https://github.com/facebook/folly/pull/1250

Reviewed By: Orvid

Differential Revision: D18222073

Pulled By: yfeldblum

fbshipit-source-id: 09cbefcc7e2ca80a95d61a4394ed74be8ea8ec55

folly/system/ThreadName.cpp

@@ -199,11 +199,11 @@ bool setThreadName(std::thread::id tid, StringPiece name) {
   if (pthread_equal(pthread_self(), id)) {
     return 0 == prctl(PR_SET_NAME, buf, 0L, 0L, 0L);
   }
-#else
+#endif
+
   (void)id;
   return false;
 #endif
-#endif
 }
 
 bool setThreadName(pthread_t pid, StringPiece name) {