net: utils: Fix unused function warning

Building with clang warns:

subsys/net/ip/utils.c:600:24: error: unused function 'pkt_calc_chksum'
[-Werror,-Wunused-function]
static inline uint16_t pkt_calc_chksum(struct net_pkt *pkt, uint16_t sum)
                       ^

pkt_calc_chksum is called by net_calc_chksum, which only exists when
CONFIG_NET_NATIVE_IP is defined.

Signed-off-by: Tom Hughes <tomhughes@chromium.org>