wic: keep rootfs_size as integer

The corrected line accidentally converted it to float,
which causes problems later on with python 3.10:

|   File "/home/alex/development/poky/scripts/lib/wic/partition.py", line 278, in prepare_rootfs_ext
|     os.ftruncate(sparse.fileno(), rootfs_size * 1024)
| TypeError: 'float' object cannot be interpreted as an integer

Signed-off-by: Alexander Kanavin <alex@linutronix.de>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>
(cherry picked from commit d1d260dd)
Signed-off-by: Steve Sakoman <steve@sakoman.com>