Support for address family constants

We already had them defined on BGP level, but they are more general.

bird-gdb.py

@@ -30,6 +30,7 @@ class BIRDFValPrinter(BIRDPrinter):
             "T_ENUM_ROA": "i",
             "T_ENUM_NETTYPE": "i",
             "T_ENUM_RA_PREFERENCE": "i",
+            "T_ENUM_AF": "i",
             "T_IP": "ip",
             "T_NET": "net",
             "T_STRING": "s",