selftests/bpf: Fix compilation error of bpf_iter_task_stack.c

BPF selftests show a compilation error as follows:

  libbpf: invalid relo for 'entries' in special section 0xfff2; forgot to
  initialize global var?..

Fix it by initializing 'entries' to zeros.

Fixes: c7568114 ("selftests/bpf: Add bpf_iter test with bpf_get_task_stack()")
Reported-by: Jesper Dangaard Brouer <brouer@redhat.com>
Signed-off-by: Song Liu <songliubraving@fb.com>
Signed-off-by: Daniel Borkmann <daniel@iogearbox.net>
Link: https://lore.kernel.org/bpf/20200703181719.3747072-1-songliubraving@fb.com