The new GetAddrInfoAsyncCancelTest cases call detail::getaddrinfo_with_timeout
directly. In split builds (make test_split) split.py moves the definition into
httplib.cc and strips `inline`, so the symbol is not declared in the public
httplib.h and test.cc fails to compile -- breaking the ubuntu/test-no-exceptions
CI jobs that the PR description says should be unaffected.
Add a forward declaration in test.cc, gated by the same #if as the tests
themselves, so it links against the split-build symbol without changing the
header-only build.