fix: [stix2 import] Fixed Observable objects types mapping

- Considering the possibility to have both types of
  IP addresses wihtin the Observed Data list of
  obervable objects

misp_stix_converter/stix2misp/converters/stix2_observable_converter.py

@@ -465,6 +465,7 @@ class ExternalSTIX2ObservableMapping(
             **dict.fromkeys(
                 (
                     'ipv4-addr',
+                    'ipv4-addr_ipv6-addr',
                     'ipv6-addr'
                 ),
                 'ip_address'