Fix kconfig warning when CONFIG_NET is not set/enabled:

WARNING: unmet direct dependencies detected for SKB_EXTENSIONS
  Depends on [n]: NET [=n]
  Selected by [y]:
  - KCOV [=y] && ARCH_HAS_KCOV [=y] && (CC_HAS_SANCOV_TRACE_PC [=y] || GCC_PLUGINS [=n])

Fixes: 6370cc3b ("net: add kcov handle to skb extensions")
Signed-off-by: Randy Dunlap <rdunlap@infradead.org>
Cc: Aleksandr Nogikh <nogikh@google.com>
Cc: Willem de Bruijn <willemb@google.com>
Link: https://lore.kernel.org/r/20201110175746.11437-1-rdunlap@infradead.orgSigned-off-by: Jakub Kicinski <kuba@kernel.org>