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: c7568114bc ("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

tools/testing/selftests/bpf/progs/bpf_iter_task_stack.c

@@ -7,7 +7,7 @@
 char _license[] SEC("license") = "GPL";
 
 #define MAX_STACK_TRACE_DEPTH   64
-unsigned long entries[MAX_STACK_TRACE_DEPTH];
+unsigned long entries[MAX_STACK_TRACE_DEPTH] = {};
 #define SIZE_OF_ULONG (sizeof(unsigned long))
 
 SEC("iter/task")