add of_str64_t string type

We need a 64-byte string for the debug_counter extension. Since adding a new
string type is so much work, the new type is named generically so that any
extension can use it.
diff --git a/c_gen/templates/loci_dump.h b/c_gen/templates/loci_dump.h
index 9cc719c..1344565 100644
--- a/c_gen/templates/loci_dump.h
+++ b/c_gen/templates/loci_dump.h
@@ -90,6 +90,7 @@
 #define LOCI_DUMP_tab_name(writer, cookie, val) LOCI_DUMP_string(writer, cookie, val)
 #define LOCI_DUMP_desc_str(writer, cookie, val) LOCI_DUMP_string(writer, cookie, val)
 #define LOCI_DUMP_ser_num(writer, cookie, val) LOCI_DUMP_string(writer, cookie, val)
+#define LOCI_DUMP_str64(writer, cookie, val) LOCI_DUMP_string(writer, cookie, val)
 
 int loci_dump_match(loci_writer_f writer, void* cookie, of_match_t *match);
 #define LOCI_DUMP_match(writer, cookie, val) loci_dump_match(writer, cookie, &val)