Fixed a compile warning in util/arena.cc when hugetlb is not supported.

Summary:
Fixed the following compile warning when hugetlb is not supported.

    ./util/arena.h:102:10: error: private field 'hugetlb_size_' is not used [-Werror,-Wunused-private-field]
      size_t hugetlb_size_ = 0;
             ^
    1 error generated.

Test Plan: make db_stress

Reviewers: igor, sdong, anthony, IslamAbdelRahman

Subscribers: dhruba, leveldb

Differential Revision: https://reviews.facebook.net/D47691

util/arena.h

@@ -99,7 +99,9 @@ class Arena : public Allocator {
   // How many bytes left in currently active block?
   size_t alloc_bytes_remaining_ = 0;
 
+#ifdef MAP_HUGETLB
   size_t hugetlb_size_ = 0;
+#endif  // MAP_HUGETLB
   char* AllocateFromHugePage(size_t bytes);
   char* AllocateFallback(size_t bytes, bool aligned);
   char* AllocateNewBlock(size_t block_bytes);