livepatch: change the error message in asm/livepatch.h header files

If anyone includes asm/livepatch.h when CONFIG_LIVEPATCH=n the build
fails with the existing error message. Change it to something saner.

[jkosina@suse.cz: fixed changelog typo spotted by Josh]
Suggested-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Miroslav Benes <mbenes@suse.cz>
Acked-by: Josh Poimboeuf <jpoimboe@redhat.com>
Signed-off-by: Jiri Kosina <jkosina@suse.cz>