selftests/bpf: Fix trampoline_count.c selftest compilation warning

Fix missing braces compilation warning in trampoline_count test:

  .../prog_tests/trampoline_count.c: In function ‘test_trampoline_count’:
  .../prog_tests/trampoline_count.c:49:9: warning: missing braces around initializer [-Wmissing-braces]
  struct inst inst[MAX_TRAMP_PROGS] = { 0 };
         ^
  .../prog_tests/trampoline_count.c:49:9: warning: (near initialization for ‘inst[0]’) [-Wmissing-braces]

Fixes: d633d579 ("selftest/bpf: Add test for allowed trampolines count")
Signed-off-by: Andrii Nakryiko <andriin@fb.com>
Signed-off-by: Daniel Borkmann <daniel@iogearbox.net>
Acked-by: Yonghong Song <yhs@fb.com>
Link: https://lore.kernel.org/bpf/20200202065152.2718142-1-andriin@fb.com