bootcount: Add include guards into bootcount.h file

This patch adds missing include guards for bootcount.h file.

Signed-off-by: Lukasz Majewski <lukma@denx.de>
Reviewed-by: Stefan Roese <sr@denx.de>
Reviewed-by: Tom Rini <trini@konsulko.com>
Reviewed-by: Alex Kiernan <alex.kiernan@gmail.com>