Adjust ssl_ciphers to be specific to OpenSSL

Syntax is OpenSSL-specific, so only use it for OpenSSL.

Discussion: https://postgr.es/m/8232E273-7B25-47F4-B0E7-3D4264106F82@yesql.se

Author: Daniel Gustafsson

Backpatch-through: head

Author: bmomjian

src/backend/utils/misc/guc.c

@@ -4124,7 +4124,7 @@ static struct config_string ConfigureNamesString[] =
 			GUC_SUPERUSER_ONLY
 		},
 		&SSLCipherSuites,
-#ifdef USE_SSL
+#ifdef USE_OPENSSL
 		"HIGH:MEDIUM:+3DES:!aNULL",
 #else
 		"none",