selftests/bpf: include limits.h needed for PATH_MAX directly

Constant PATH_MAX is used in function unpriv_helpers.c:open_config().
This constant is provided by include file <limits.h>.
The dependency was added by commit [1], which does not include
<limits.h> directly, relying instead on <limits.h> being included from
zlib.h -> zconf.h.
As it turns out, this is not the case for all systems, e.g. on
Fedora 41 zlib 1.3.1 is used, and there <limits.h> is not included
from zconf.h. Hence, there is a compilation error on Fedora 41.

[1] commit fc2915bb8b ("selftests/bpf: More precise cpu_mitigations state detection")

Fixes: fc2915bb8b ("selftests/bpf: More precise cpu_mitigations state detection")
Signed-off-by: Eduard Zingerman <eddyz87@gmail.com>
Acked-by: Viktor Malik <vmalik@redhat.com>
Link: https://lore.kernel.org/r/20250618093134.3078870-1-eddyz87@gmail.com
Signed-off-by: Alexei Starovoitov <ast@kernel.org>

tools/testing/selftests/bpf/unpriv_helpers.c

@@ -1,6 +1,7 @@
 // SPDX-License-Identifier: GPL-2.0-only
 
 #include <errno.h>
+#include <limits.h>
 #include <stdbool.h>
 #include <stdlib.h>
 #include <stdio.h>