tools: mkimage: make size_inc a signed type

tools: mkimage: make size_inc a signed type

In the Fixes commit, I initialized size_inc from the return value of
the new fit_estimate_hash_sig_size() helper. That helper may fail and
report that by returning a negative value, but I overlooked that
size_inc had type size_t, and hence the error check doesn't work.

Change size_inc to have type int so the error check works.  Inside the
loop, it is passed to another function as a size_t parameter, but
that's fine, because we know it is non-negative, and its value may be
incremented in steps of 1024 and is capped at ~64K, so it will
certainly never overflow an int.

Fixes: 7d4eacb0e68 ("mkimage: do a rough estimate for the size needed for hashes/signatures")
Addresses-Coverity-ID: 569495: Integer handling issues  (NEGATIVE_RETURNS)
Signed-off-by: Rasmus Villemoes <ravi@prevas.dk>