Avoid duplicate definition of git_http_auth_dummy

Avoid defining `git_http_auth_ntlm` when not using ntlm; this will be
set to a dummy function instead.

src/libgit2/transports/auth_sspi.c

@@ -333,11 +333,13 @@ int git_http_auth_negotiate(
 }
 #endif
 
+#ifdef GIT_AUTH_NTLM
 int git_http_auth_ntlm(
 	git_http_auth_context **out,
 	const git_net_url *url)
 {
 	return sspi_init_context(out, GIT_HTTP_AUTH_NTLM, url);
 }
+#endif
 
 #endif /* GIT_WIN32 */