Remove -Wno-unused-parameter

Introduced UNUSED macro with cast to void in commoh.h for internal
use. Used cast to void directly in those files which do not
include common.h. Although this change doesn't fix semantic issues
with unused function parameters, it does explicitly mark all those
places, which might require attention in future.

examples/basic.c

@@ -28,6 +28,9 @@ void conn_handler(xmpp_conn_t *const conn,
     xmpp_ctx_t *ctx = (xmpp_ctx_t *)userdata;
     int secured;
 
+    (void)error;
+    (void)stream_error;
+
     if (status == XMPP_CONN_CONNECT) {
         fprintf(stderr, "DEBUG: connected\n");
         secured = xmpp_conn_is_secured(conn);