sms-part-3gpp: fix warnings with -Wswitch-default

mm-sms-part-3gpp.c: In function ‘sms_encoding_type’:
  mm-sms-part-3gpp.c:190:13: error: switch missing default case [-Werror=switch-default]
    190 |             switch (dcs & 0x0c) {
        |             ^~~~~~
  mm-sms-part-3gpp.c:219:13: error: switch missing default case [-Werror=switch-default]
    219 |             switch (dcs & 0x04) {
        |             ^~~~~~

src/mm-sms-part-3gpp.c

@@ -192,14 +192,16 @@ sms_encoding_type (int dcs)
                     scheme = MM_SMS_ENCODING_UCS2;
                     break;
                 case 0x00:
-                    /* fallthrough */
                     /* reserved - spec says to treat it as default alphabet */
+                    /* Fall through */
                 case 0x0c:
                     scheme = MM_SMS_ENCODING_GSM7;
                     break;
                 case 0x04:
                     scheme = MM_SMS_ENCODING_8BIT;
                     break;
+                default:
+                    g_assert_not_reached ();
             }
             break;
 
@@ -223,6 +225,8 @@ sms_encoding_type (int dcs)
                 case 0x04:
                     scheme = MM_SMS_ENCODING_8BIT;
                     break;
+                default:
+                    g_assert_not_reached ();
             }
             break;
 
@@ -689,6 +693,8 @@ mm_sms_part_3gpp_new_from_binary_pdu (guint index,
                     mm_sms_part_set_concat_max (sms_part,pdu[offset + 2]);
                     mm_sms_part_set_concat_sequence (sms_part, pdu[offset + 3]);
                     break;
+                default:
+                    break;
                 }
 
                 offset += ie_len;