sysutils/flashrom: fix build on big-endian

compare_region_with_dump() is used only in a code for little-endian:
../libflashrom.c:458:12: error: unused function 'compare_region_with_dump' [-Werror,-Wunused-function]

458 | static int compare_region_with_dump(const struct romentry *const a, const struct romentry *const b)
    |            ^~~~~~~~~~~~~~~~~~~~~~~~

1 error generated.