Give struct vpi_ref a magic

to be able to use our standard miniobj facilities on it, PAN_dump_struct
in this case.

As vpi_ref is static const in VGC, there is no real reason for magic
checks other than that.

include/vcc_interface.h

@@ -46,6 +46,8 @@ void v_noreturn_ VPI_Fail(const char *func, const char *file, int line,
  */
 
 struct vpi_ref {
+	unsigned	magic;
+#define VPI_REF_MAGIC	0xd955f567
 	unsigned	source;
 	unsigned	offset;
 	unsigned	line;

lib/libvcc/vcc_compile.c

@@ -410,7 +410,7 @@ EmitCoordinates(const struct vcc *tl, struct vsb *vsb)
 				pos++;
 
 		}
-		VSB_printf(vsb, "  [%3u] = { %u, %8tu, %4u, %3u, ",
+		VSB_printf(vsb, "  [%3u] = { VPI_REF_MAGIC, %u, %8tu, %4u, %3u, ",
 		    t->cnt, sp->idx, t->b - sp->b, lin, pos + 1);
 		if (t->tok == CSRC)
 			VSB_cat(vsb, " \"C{\"},\n");