Fix build with LibreSSL (following commit 537f37898)

Hello,

This patch fixes the following build errors with LibreSSL (3.1.4) :

/bin/sh ../libtool --silent  --tag=CXX   --mode=compile c++ -DHAVE_CONFIG_H -I. -I../lib  -I../lib -I../trio  -I/usr/include  -DLIBICONV_PLUG -I/usr/local/include -isystem /usr/local/include -D_THREAD_SAFE  -O2 -pipe -DLIBICONV_PLUG -fstack-protector-strong -isystem /usr/local/include -fno-strict-aliasing  -DLIBICONV_PLUG -isystem /usr/local/include  -Wall -Wwrite-strings -Woverloaded-virtual -fno-exceptions -fno-rtti -fno-implement-inlines -MT liblftp_network_la-lftp_ssl.lo -MD -MP -MF .deps/liblftp_network_la-lftp_ssl.Tpo -c -o liblftp_network_la-lftp_ssl.lo `test -f 'lftp_ssl.cc' || echo './'`lftp_ssl.cc
lftp_ssl.cc:1164:10: error: use of undeclared identifier 'X509_STORE_CTX_get_by_subject'; did you mean 'X509_STORE_get_by_subject'?
    rc = X509_STORE_CTX_get_by_subject(store_ctx, X509_LU_CRL, subject, obj);
         ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~
         X509_STORE_get_by_subject
/usr/local/include/openssl/x509_vfy.h:479:5: note: 'X509_STORE_get_by_subject' declared here
int X509_STORE_get_by_subject(X509_STORE_CTX *vs,int type,X509_NAME *name,
    ^
lftp_ssl.cc:1204:10: error: use of undeclared identifier 'X509_STORE_CTX_get_by_subject'; did you mean 'X509_STORE_get_by_subject'?
    rc = X509_STORE_CTX_get_by_subject(store_ctx, X509_LU_CRL, issuer, obj);
         ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~
         X509_STORE_get_by_subject
/usr/local/include/openssl/x509_vfy.h:479:5: note: 'X509_STORE_get_by_subject' declared here
int X509_STORE_get_by_subject(X509_STORE_CTX *vs,int type,X509_NAME *name,
    ^
2 errors generated.
*** Error code 1

lavv17 / lftp

Fix build with LibreSSL (following commit 537f37898) #602