kwbimage.c: Fix compile warning when building on 64 bit systems

Fix this warning when building on 64 bit systems:
tools/kwbimage.c: In function 'kwbimage_checksum32':
tools/kwbimage.c:135: warning: format '%d' expects type 'int',
but argument 4 has type 'long unsigned int'

Signed-off-by: Wolfgang Denk <wd@denx.de>
Cc: Prafulla Wadaskar <prafulla@marvell.com>