Make xtables_target->extra_opts const (xtables_match->extra_opts already is)

Signed-off-by: Jan Engelhardt <jengelh@gmx.de>
diff --git a/include/xtables.h b/include/xtables.h
index 231f514..72e6ae2 100644
--- a/include/xtables.h
+++ b/include/xtables.h
@@ -158,7 +158,7 @@
 		     const struct xt_entry_target *target);
 
 	/* Pointer to list of extra command-line options */
-	struct option *extra_opts;
+	const struct option *extra_opts;
 
 	/* Ignore these men behind the curtain: */
 	unsigned int option_offset;