net: Fix Kconfig warning

The double include of Kconfig.debug causes the following warning:

scripts/kconfig/conf --silentoldconfig Kconfig
/home/jh/src/zephyr/net/ip/Kconfig.debug:45:warning: choice value used outside its choice group
/home/jh/src/zephyr/net/ip/Kconfig.debug:47:warning: choice value used outside its choice group
/home/jh/src/zephyr/net/ip/Kconfig.debug:49:warning: choice value used outside its choice group

Change-Id: I3f59587eed656b67e1f69e2a8151288f41b50769
Signed-off-by: Johan Hedberg <johan.hedberg@intel.com>