bpf: handle implicit declaration of function gettid in bpf_iter.c

bpf: handle implicit declaration of function gettid in bpf_iter.c

As we can see from the title, when I compiled the selftests/bpf, I
saw the error:
implicit declaration of function ‘gettid’ ; did you mean ‘getgid’? [-Werror=implicit-function-declaration]
  skel->bss->tid = gettid();
                   ^~~~~~
                   getgid

Directly call the syscall solves this issue.

Signed-off-by: Jason Xing <kernelxing@tencent.com>
Reviewed-by: Alan Maguire <alan.maguire@oracle.com>
Tested-by: Alan Maguire <alan.maguire@oracle.com>
Link: https://lore.kernel.org/r/20241029074627.80289-1-kerneljasonxing@gmail.com
Signed-off-by: Martin KaFai Lau <martin.lau@kernel.org>