fix build without gflags

Test Plan:
Built and ran with gflags:
% ./db_bench
LevelDB:    version 4.5
Date:       Tue Feb 16 12:04:23 2016
CPU:        40 * Intel(R) Xeon(R) CPU E5-2660 v2 @ 2.20GHz
...

And without gflags:
% ./db_bench
Please install gflags to run rocksdb tools
%

Reviewers: sdong, igor

Reviewed By: igor

Subscribers: igor, dhruba

Differential Revision: https://reviews.facebook.net/D54243

tools/db_bench_tool.cc

@@ -11,6 +11,7 @@
 #define __STDC_FORMAT_MACROS
 #endif
 
+#ifdef GFLAGS
 #ifdef NUMA
 #include <numa.h>
 #include <numaif.h>
@@ -4132,3 +4133,4 @@ int db_bench_tool(int argc, char** argv) {
   return 0;
 }
 }  // namespace rocksdb
+#endif