FreeBSD: skip checking of xktls_session* structs on FreeBSD 14.x and older

(backport <#4482>)
(cherry picked from commit db4dba7)

Author: kostikbel

libc-test/build.rs

@@ -2920,6 +2920,10 @@ fn test_freebsd(target: &str) {
             // `splice` introduced in FreeBSD 14.2
             "splice" if Some(14) > freebsd_ver => true,
 
+            // Those are introduced in FreeBSD 15.
+            "xktls_session_onedir" | "xktls_session"
+                if Some(15) > freebsd_ver => true,
+
             _ => false,
         }
     });