libc-test: Account for xktls_session_onedir::gen (freebsd)

(backport <#4552>)
(cherry picked from commit 56a04ab)

Author: neuschaefer

libc-test/build.rs

@@ -2585,6 +2585,8 @@ fn test_freebsd(target: &str) {
             "type_" if struct_ == "sockstat" => "type".to_string(),
             "type_" if struct_ == "devstat_match_table" => "type".to_string(),
             "type_" if struct_ == "input_event" => "type".to_string(),
+            // Field is named `gennum` in Rust because `gen` is a keyword
+            "gennum" if struct_ == "xktls_session_onedir" => "gen".to_string(),
             s => s.to_string(),
         }
     });