serial_core.h: include <linux/sysrq.h>

The <linux/serial_core.h> header refers to handle_sysrq(), but does not
include <linux/sysrq.h> which provides a declaration of the function.  This
may result in an implicit declaration and a warning if the actual one is
seen later on.

Signed-off-by: Maciej W. Rozycki <macro@linux-mips.org>
Cc: Russell King <rmk@arm.linux.org.uk>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>