samples: bpf: add ibumad sample to .gitignore

This commit adds ibumad to .gitignore which is
currently ommited from the ignore file.
Signed-off-by: Matteo Croce <mcroce@redhat.com>
Signed-off-by: Alexei Starovoitov <ast@kernel.org>

samples/bpf/.gitignore

 cpustat
 fds_example
 hbm
+ibumad
 lathist
 lwt_len_hist
 map_perf_test