nand-utils: nanddump: Fix unexpected type compiling warning for argument 'dumpfile'

nand-utils: nanddump: Fix unexpected type compiling warning for argument 'dumpfile'

The const pointer argument cannot be passed into libc function 'free()',
otherwise the compiler will complain following message:
 nand-utils/nanddump.c:168:10: warning: passing argument 1 of ‘free’
 discards ‘const’ qualifier from pointer target type

Fixes: c89009463888 ("nanddump: don't leak copied command line arguments")
Signed-off-by: Zhihao Cheng <chengzhihao1@huawei.com>
Signed-off-by: David Oberhollenzer <david.oberhollenzer@sigma-star.at>