wic: plugins: rawcopy: Fixed wrong variable type

wic: plugins: rawcopy: Fixed wrong variable type

Without the int() function this variable will be a string. This will led
to a error in Filemap on line 545 due wrong types.

> [...]
>   File
> ".../poky/scripts/lib/wic/filemap.py", line 545, in sparse_copy
>     if start < skip < end:
> TypeError: unorderable types: int() < str()

Signed-off-by: Daniel Schultz <d.schultz@phytec.de>
Signed-off-by: Ross Burton <ross.burton@intel.com>