linux/sysrq.h needs linux/errno.h

In include/linux/sysrq.h the constant EINVAL is being used but is undefined
if include/linux/errno.h is not included before.

Fix this by adding #include <linux/errno.h> at the beginning.

Signed-off-by: Tobias Doerffel <tobias.doerffel@gmail.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>