selftests: drv-net: fix missing include in ncdevmem

Commit ca9d74eb5f6a ("uapi: add INT_MAX and INT_MIN constants")
recently removed some includes of limits.h in uAPI headers.
ncdevmem.c was depending on them:

ncdevmem.c: In function ‘ethtool_add_flow’:
ncdevmem.c:369:60: error: ‘INT_MAX’ undeclared (first use in this function)
369 | if (endptr == id_start || flow_id < 0 || flow_id > INT_MAX)
| ^~~~~~~
ncdevmem.c:77:1: note: ‘INT_MAX’ is defined in header ‘<limits.h>’; did you forget to ‘#include <limits.h>’?

Reviewed-by: Mina Almasry <almasrymina@google.com>
Link: https://patch.msgid.link/20260120180319.1673271-1-kuba@kernel.org
Signed-off-by: Jakub Kicinski <kuba@kernel.org>