selftests/bpf: Fix fclose/pclose mismatch in test_progs

Make sure to use pclose() to properly close the pipe opened by popen().

Fixes: 81f77fd0

 ("bpf: add selftest for stackmap with BPF_F_STACK_BUILD_ID")
Signed-off-by: Andrea Righi <andrea.righi@canonical.com>
Signed-off-by: Daniel Borkmann <daniel@iogearbox.net>
Reviewed-by: Shuah Khan <skhan@linuxfoundation.org>
Acked-by: Martin KaFai Lau <kafai@fb.com>
Link: https://lore.kernel.org/bpf/20211026143409.42666-1-andrea.righi@canonical.com