port: increase padding of struct port

We are going to extend context of struct port_sql. One already inherits
struct port_tuple, which makes it size barely fits into 48 bytes of
padding of basic structure (struct port). Hence, let's increase padding
a bit to be able to add at least one more member to struct port_sql.

Needed for #2592

src/lib/core/port.h

@@ -122,7 +122,7 @@ struct port {
 	 * Implementation dependent content. Needed to declare
 	 * an abstract port instance on stack.
 	 */
-	char pad[48];
+	char pad[52];
 };
 
 /** Is not inlined just to be exported. */