Silence two "unused function af_lookup" warnings (#5222)

Silence two "unused function af_lookup" warnings (#5222)

Gcc7 is smarter about detecting unused functions and detects those two functions
which are unused in tests. But gperf generates them for us, so let's instead of removing
tell gcc that we know they might be unused in the test code.

In file included from ../src/test/test-af-list.c:29:0:
./src/basic/af-from-name.h:140:1: warning: ‘lookup_af’ defined but not used [-Wunused-function]
 lookup_af (register const char *str, register size_t len)
 ^~~~~~~~~
In file included from ../src/test/test-arphrd-list.c:29:0:
./src/basic/arphrd-from-name.h:125:1: warning: ‘lookup_arphrd’ defined but not used [-Wunused-function]
 lookup_arphrd (register const char *str, register size_t len)
 ^~~~~~~~~~~~~