ktls.h: Guard includes behind _KERNEL

These are not needed when including ktls.h to get sockopt definitions.

Reviewed by:	gallatin, jhb
MFC after:	1 week
Sponsored by:	The FreeBSD Foundation
Differential Revision:	https://reviews.freebsd.org/D30392

sys/sys/ktls.h

@@ -29,8 +29,10 @@
 #ifndef _SYS_KTLS_H_
 #define	_SYS_KTLS_H_
 
+#ifdef _KERNEL
 #include <sys/refcount.h>
 #include <sys/_task.h>
+#endif
 
 struct tls_record_layer {
 	uint8_t  tls_type;