mkimage: struct stat.st_size may not be long

The component st_size of struct stat is of type off_t. Depending on the
system printing it it with %ld leads to a warning:

tools/mkimage.c:438:54: warning: format '%ld' expects argument of type
'long int', but argument 5 has type
'off_t' {aka 'long long int'} [-Wformat=]
  438 |     "%s: Bad size: \"%s\" is not valid image: size %ld < %u\n",
      |                                                    ~~^
      |                                                      |
      |                                                      long int
      |                                                    %lld

When comparing an off_t value to a 32bit integer we should not convert to
uint32_t but to off_t which may be wider.

Reported-by: Milan P. Stanić <mps@arvanta.net>
Fixes: 331f0800 ("mkimage: allow -l to work on block devices on Linux")
Signed-off-by: Heinrich Schuchardt <heinrich.schuchardt@canonical.com>
Reviewed-by: Mark Kettenis <kettenis@openbsd.org>