csum-file: define hashwrite's count as a uint32_t

csum-file: define hashwrite's count as a uint32_t

We want to call this code from Rust and ensure that the types are the
same for compatibility, which is easiest to do if the type is a fixed
size.  Since unsigned int is 32 bits on all the platforms we care about,
define it as a uint32_t instead.

Signed-off-by: brian m. carlson <sandals@crustytoothpaste.net>
Signed-off-by: Junio C Hamano <gitster@pobox.com>