Added missing component : gflags in Linux platform.

In INSTALL.md, installation command of gflags for Linux platform is missing.
Without it, users will get the following error while compiling db/db_bench.cc.

db/db_bench.cc:14:27: fatal error: gflags/gflags.h: No such file or directory
#include <gflags/gflags.h>
                           ^