firmware-utils: mkfwimage: fix build failure on macOS with gcc 9

firmware-utils: mkfwimage: fix build failure on macOS with gcc 9

Fixes following errors:

 src/mkfwimage.c:279:8: error: format specifies type 'long' but the argument has type 'off_t' (aka 'long long') [-Werror,-Wformat]
                      d->stats.st_size,
                     ^~~~~~~~~~~~~~~~
 src/mkfwimage.c:280:8: error: format specifies type 'long' but the argument has type 'long long' [-Werror,-Wformat]
                      d->partition_length - d->stats.st_size);
                      ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
 src/mkfwimage.c:378:6: error: format specifies type 'unsigned long' but the argument has type 'long long' [-Werror,-Wformat]
                      d->stats.st_size - d->partition_length);
                      ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~

Reported-by: Kevin Darbyshire-Bryant <ldir@darbyshire-bryant.me.uk>
Signed-off-by: Petr Štetiar <ynezz@true.cz>