[libcasper] Use null inline functions when casper is disabled

[adrian]

Summary

• use null functions rather than defines to make sure the compiler doesn't complain about unused function arguments, which gcc-6 on mips32 indeed does!

Test Plan

• compiled for freebsd wifi mips testing where i actively disable this stuff to save space

[emaste]

Agree with the change, two small suggestions

lib/libcasper/services/cap_dns/cap_dns.h

47–48 ↗

(On Diff #78177)

Maybe style disagrees but I'd probably just put these in the #else case below.

81 ↗

(On Diff #78177)

maybe

static inline struct hostent *
cap_gethostbyname(cap_channel_t *chan __unused, const char *name)