os/bluestore: silence gcc warning

os/bluestore: silence gcc warning

silences following warning:

src/os/bluestore/BitAllocator.h:410:8: warning: ‘virtual bool
BitMapAreaIN::child_check_n_lock(BitMapArea*, int64_t, bool)’ was hidden
[-Wover\
loaded-virtual]
   bool child_check_n_lock(BitMapArea *child, int64_t required, bool
lock) {
        ^~~~~~~~~~~~~~~~~~
/var/ceph/ceph/src/os/bluestore/BitAllocator.h:489:8: warning:   by
‘BitMapAreaLeaf::child_check_n_lock’ [-Woverloaded-virtual]
   bool child_check_n_lock(BitMapZone* child, int64_t required, bool
lock);
        ^~~~~~~~~~~~~~~~~~

Signed-off-by: Kefu Chai <kchai@redhat.com>