net: udp: replace net_udp_get/set_hdr macros with static inline

Replace net_udp_get_hdr and net_udp_set_hdr macros with static inline
function definitions to avoid unused variable build warnings when
NET_UDP is not defined.

This fixes the following warning:

subsys/net/ip/6lo.c: In function 'compress_IPHC_header':
subsys/net/ip/6lo.c:759:22: warning: unused variable 'hdr' [-Wunused-variable]
   struct net_udp_hdr hdr, *udp;
                      ^~~

Signed-off-by: Ricardo Salveti <ricardo.salveti@linaro.org>