The function bpf_get_stackid is defined in helpers.h:
  int bpf_get_stackid(uintptr_t map, void *ctx, u64 flags)

But the same function is also defined in
linux:include/linux/bpf.h:
  u64 bpf_get_stackid(u64 r1, u64 r2, u64 r3, u64 r4, u64 r5);

If a bpf program also includes, directly or indirectly,
linux/bpf.h, compilation will fail because incompatible
definition.

This patch renames bcc helpers.h definition to
bcc_get_stackid to avoid this issue.
Signed-off-by: Yonghong Song <yhs@fb.com>