Block forward_iterator_bench under MAC and Windows

Summary:
Travis is now failing because we cannot compile forward_iterator_bench under MAC
https://travis-ci.org/facebook/rocksdb/jobs/91524025

In forward_iterator_bench.cc we are using multiple functions that are not available in MAC like
htobe64
be64toh

Blocking forward_iterator_bench under MAC

Test Plan: compile under mac

Reviewers: rven, yhchiang, anthony, sdong

Reviewed By: sdong

Subscribers: dhruba

Differential Revision: https://reviews.facebook.net/D50889

db/forward_iterator_bench.cc

@@ -13,6 +13,9 @@ int main() {
   fprintf(stderr, "Please install gflags to run rocksdb tools\n");
   return 1;
 }
+#elif defined(OS_MACOSX) || defined(OS_WIN)
+// Block forward_iterator_bench under MAC and Windows
+int main() { return 0; }
 #else
 #include <gflags/gflags.h>
 #include <semaphore.h>