tests: checkfs: Add previous prototype for do_pwr_dn()

tests: checkfs: Add previous prototype for do_pwr_dn()

The compiler complains following message:
 tests/checkfs/comm.c:37:5: warning: no previous prototype for
 ‘do_pwr_dn’
The function do_pwr_dn() is only used in tests/checkfs/checkfs.c, but
we still keep it in tests/checkfs/comm.c in case the function() is
used by more callers. Fix it by adding a previous declaration.

Signed-off-by: Zhihao Cheng <chengzhihao1@huawei.com>
Signed-off-by: David Oberhollenzer <david.oberhollenzer@sigma-star.at>