tls.c:994:2: error: implicit declaration of function 'SSL_CTX_set1_cert_store'

Job #2403711 failed for bb61a3a9 on Debian 9 (and only there):

tls.c: In function 'isc_tlsctx_enable_peer_verification':
tls.c:994:2: error: implicit declaration of function 'SSL_CTX_set1_cert_store' [-Werror=implicit-function-declaration]
  SSL_CTX_set1_cert_store(tlsctx, store);
  ^~~~~~~~~~~~~~~~~~~~~~~

This code originated in c49a81e2.