selftests: drivers: net: fix name not defined

This fix solves this error, when calling kselftest with targets
"drivers/net":

File "tools/testing/selftests/net/lib/py/nsim.py", line 64, in __init__
  if e.errno == errno.ENOSPC:
NameError: name 'errno' is not defined

The error was found by running tests manually with the command:
make kselftest TARGETS="drivers/net"

The module errno makes available standard error system symbols.
Reviewed-by: Petr Machata <petrm@nvidia.com>
Signed-off-by: Alessandro Zanni <alessandro.zanni87@gmail.com>
Link: https://patch.msgid.link/20241010183034.24739-1-alessandro.zanni87@gmail.comSigned-off-by: Jakub Kicinski <kuba@kernel.org>

tools/testing/selftests/net/lib/py/nsim.py

 # SPDX-License-Identifier: GPL-2.0
 
+import errno
 import json
 import os
 import random