net: vlan: Use IS_ENABLED() helper for CONFIG_VLAN_8021Q guard

net: vlan: Use IS_ENABLED() helper for CONFIG_VLAN_8021Q guard

The header currently tests the VLAN core with an explicit pair of 'if
defined' checks:
    #if defined(CONFIG_VLAN_8021Q) || defined(CONFIG_VLAN_8021Q_MODULE)

Instead, use IS_ENABLED() which is the kernel way to test whether an
option is configured as builtin/module.

This is purely cosmetic – no functional changes.

Reviewed-by: Alex Lazar <alazar@nvidia.com>
Reviewed-by: Dragos Tatulea <dtatulea@nvidia.com>
Signed-off-by: Gal Pressman <gal@nvidia.com>
Link: https://patch.msgid.link/20250616132626.1749331-4-gal@nvidia.com
Signed-off-by: Jakub Kicinski <kuba@kernel.org>