Add dummy PQsslAttributes function for non-SSL builds.

All the other new SSL information functions had dummy versions in
be-secure.c, but I missed PQsslAttributes(). Oops. Surprisingly, the linker
did not complain about the missing function on most platforms represented in
the buildfarm, even though it is exported, except for a few Windows systems.

src/interfaces/libpq/fe-secure.c

@@ -407,6 +407,14 @@ PQsslAttribute(PGconn *conn, const char *attribute_name)
 {
 	return NULL;
 }
+
+const char **
+PQsslAttributes(PGconn *conn)
+{
+	static const char *result[] = { NULL };
+
+	return result;
+}
 #endif   /* USE_SSL */