fix #warning about using kernel headers in userpsace

Move the preprocessor #warning message:
warning: #warning Attempt to use kernel headers from user space,
see http://kernelnewbies.org/KernelHeaders


from kernel.h to types.h.

And also fixe the #warning message due to the preprocessor not being able to
read the web address due to it thinking it was the start of a comment.  also
remove the extra #ifndef _KERNEL_ since it's already there.

Signed-off-by: Justin P. Mattock <justinmattock@gmail.com>
Cc: Arnd Bergmann <arnd@arndb.de>
Signed-off-by: Jiri Kosina <jkosina@suse.cz>