selftests/bpf: Use pid_t consistently in test_progs.c

Use pid_t rather than __pid_t when allocating memory for 'worker_pids' in
'struct test_env', as this is its declared type and also avoids compile
errors seen building against musl libc on mipsel64:

  test_progs.c:1738:49: error: '__pid_t' undeclared (first use in this function); did you mean 'pid_t'?
   1738 |                 env.worker_pids = calloc(sizeof(__pid_t), env.workers);
        |                                                 ^~~~~~~
        |                                                 pid_t
  test_progs.c:1738:49: note: each undeclared identifier is reported only once for each function it appears in

Fixes: 91b2c0af ("selftests/bpf: Add parallelism to test_progs")
Signed-off-by: Tony Ambardar <tony.ambardar@gmail.com>
Signed-off-by: Andrii Nakryiko <andrii@kernel.org>
Acked-by: Geliang Tang <geliang@kernel.org>
Link: https://lore.kernel.org/bpf/c6447da51a94babc1931711a43e2ceecb135c93d.1721713597.git.tony.ambardar@gmail.com