hnbap_common: hnbap_cause_str: handle nothing val

Fix for:
../../../src/osmo-iuh/src/hnbap_common.c:85:9: error: enumeration value ‘HNBAP_Cause_PR_NOTHING’ not handled in switch [-Werror=switch]

Change-Id: Icab3052e27d1951129e3809e61357ee23c4cb588

src/hnbap_common.c

@@ -103,6 +103,9 @@ char *hnbap_cause_str(HNBAP_Cause_t *cause)
 			get_value_string(hnbap_cause_misc_vals,
 					cause->choice.misc));
 		break;
+	case HNBAP_Cause_PR_NOTHING:
+		snprintf(buf, sizeof(buf), "NOTHING");
+		break;
 	}
 	return buf;
 }