samples: bpf: add hbm sample to .gitignore

This commit adds hbm to .gitignore which is
currently ommited from the ignore file.
Signed-off-by: Daniel T. Lee <danieltimlee@gmail.com>
Signed-off-by: Daniel Borkmann <daniel@iogearbox.net>

samples/bpf/.gitignore

 cpustat
 fds_example
+hbm
 lathist
 lwt_len_hist
 map_perf_test