ss_bsdiff: Add casting for explicit conversion

ss_bsdiff: Add casting for explicit conversion

Since there were implicit conversion from off_t to uint64_t, which
caused by the compariton between off_t type variables and uint64_t type
varaibles.

It causes warning about the comparison between unmatched types thus the
off_t value will be casted implicitly to uint64_t if this warning is
ignored.

To suppress this warning and preserve the behavior of the original code,
explicit type casting to uint64_t is added.

Change-Id: Ieb6c0e073986e2cf0ef686307b2cfe894d84f27d
Signed-off-by: SangYoun Kwak <sy.kwak@samsung.com>