libc-test: include sys/ktls.h on freebsd

<sys/ktls.h> is necessary in order to find the xktls_* structs.

See also:

freebsd/freebsd-src@c9e9a0f

(backport <rust-lang#4552>)
(cherry picked from commit 4147a8b)

Author: neuschaefer

libc-test/build.rs

@@ -2503,7 +2503,8 @@ fn test_freebsd(target: &str) {
                 "sys/shm.h",
                 "sys/socket.h",
                 "sys/socketvar.h",
-                "netinet/in_pcb.h",	// must be after sys/socketvar.h
+                [freebsd15]:"sys/ktls.h",
+                "netinet/in_pcb.h",	// must be after sys/socketvar.h, sys/ktls.h
                 "sys/stat.h",
                 "sys/statvfs.h",
                 "sys/sysctl.h",