Use forward.h instead of declaring structures
diff --git a/options.h b/options.h
index 3ffee71..0806928 100644
--- a/options.h
+++ b/options.h
@@ -1,7 +1,7 @@
 #include <stdio.h>
 #include <sys/types.h>
 
-struct filter;
+#include "forward.h"
 
 struct options_t {
 	int align;      /* -a: default alignment column for results */