BUILD: ssl: fine guard for SSL_CTX_get0_privatekey call

SSL_CTX_get0_privatekey is openssl/boringssl specific function present
since openssl-1.0.2, let us define readable guard for it, not depending
on HA_OPENSSL_VERSION

(cherry picked from commit af204881a3df36da1451af33f57b2c11ecb0972e)
Signed-off-by: Christopher Faulet <cfaulet@haproxy.com>
diff --git a/src/ssl_sock.c b/src/ssl_sock.c
index 9456b7a..5968a34 100644
--- a/src/ssl_sock.c
+++ b/src/ssl_sock.c
@@ -1878,7 +1878,7 @@
 	int 	      key_type;
 
 	/* Get the private key of the default certificate and use it */
-#if (HA_OPENSSL_VERSION_NUMBER >= 0x10002000L)
+#ifdef HAVE_SSL_CTX_get0_privatekey
 	pkey = SSL_CTX_get0_privatekey(bind_conf->default_ctx);
 #else
 	tmp_ssl = SSL_new(bind_conf->default_ctx);