Hide tunable definitions behind _KERNEL

Some userspace code include sys/kernel.h. Namely, some OpenZFS tests do
this, and it was causing breakage after r365945 due to a lack of bool
typedef. Userspace should not need the TUNABLE_** stuff, so hide it
behind an #ifdef _KERNEL.

Sorry for the breakage.

Reported by:	andrew, Michael Butler, Jenkins
Discussed with: kevans, allanjude

sys/sys/kernel.h

@@ -297,6 +297,8 @@ sysinit_tslog_shim(const void * data)
 
 void	sysinit_add(struct sysinit **set, struct sysinit **set_end);
 
+#ifdef _KERNEL
+
 /*
  * Infrastructure for tunable 'constants'.  Value may be specified at compile
  * time or kernel load time.  Rules relating tunables together can be placed
@@ -459,6 +461,8 @@ struct tunable_str {
 #define	TUNABLE_STR_FETCH(path, var, size)			\
 	getenv_string((path), (var), (size))
 
+#endif /* _KERNEL */
+
 typedef void (*ich_func_t)(void *_arg);
 
 struct intr_config_hook {