os/bluestore/BlueStore: make maybe unused variable [[maybe_unused]]

os/bluestore/BlueStore: make maybe unused variable [[maybe_unused]]

cct is defined so it can be used by dout(), but when the source code is
compiled for crimson, dout() does not use cct at all, so in that case,
compiler complains like:

../src/os/bluestore/BlueStore.cc:17413:8: warning: unused variable 'cct' [-Wunused-variable]
  auto cct  = onode->c->store->cct;
       ^
1 warning generated.

in this change, cct is marked with [[maybe_unused]] to silence this
warning.

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