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/java_gen/java_type.py b/java_gen/java_type.py
index 6188771..be8251b 100644
--- a/java_gen/java_type.py
+++ b/java_gen/java_type.py
@@ -349,6 +349,7 @@
 desc_str = gen_fixed_length_string_jtype(256)
 serial_num = gen_fixed_length_string_jtype(32)
 table_name = gen_fixed_length_string_jtype(32)
+str64 = gen_fixed_length_string_jtype(64)
 ipv4 = JType("IPv4Address") \
         .op(read="IPv4Address.read4Bytes(bb)", \
             write="$name.write4Bytes(bb)",
@@ -526,6 +527,7 @@
         'of_serial_num_t': serial_num,
         'of_port_name_t': port_name,
         'of_table_name_t': table_name,
+        'of_str64_t': str64,
         'of_ipv4_t': ipv4,
         'of_ipv6_t': ipv6,
         'of_wc_bmap_t': flow_wildcards,