Use static inline for iface_{setup,delete}_addr in tests/sys/net/routing.

This fixes possible link errors, similar to:

ld: error: undefined symbol: iface_setup_addr

referenced by test_rtsock_l3.c:111 (tests/sys/net/routing/test_rtsock_l3.c:111)

test_rtsock_l3.o:(presetup_ipv4)

referenced by test_rtsock_l3.c:79 (tests/sys/net/routing/test_rtsock_l3.c:79)

test_rtsock_l3.o:(presetup_ipv6)

referenced by test_rtsock_l3.c:512 (tests/sys/net/routing/test_rtsock_l3.c:512)

test_rtsock_l3.o:(atfu_rtm_change_v4_gw_success_body)

referenced 10 more times

In C (not C++), 'naked' inline is almost always a mistake. Either use
static inline (this is appropriate for most cases), or extern inline.

MFC after: 3 days