Fix compilation with --with-gssapi --without-openssl
The #define is spelled ENABLE_GSS, not USE_GSS. Introduced in commit
05fd30c0e7, reported by Thomas Munro.
Discussion: https://www.postgresql.org/message-id/CA%2BhUKG%2BHRTtB%2Bx%2BKKKj_cfX6sNhbeGuqmGxjGMwdVPG7YGFP8w@mail.gmail.com