ftp/curl: Fix build with --with-gssapi and --disable-ftp

Approved by: jrm (mentor), otis (mentor), sunpoet (implicit approval)
Differential Revision: https://reviews.freebsd.org/D50052
Upstream PR: https://github.com/curl/curl/issues/16925

(cherry picked from commit b55113e4038c74da6ab611a932db9cb041b7b967)