Fix more fallout for bundled rocksdb after the LLVM 10 import

In file included from rocksdb/utilities/backupable/backupable_db.cc:16:
rocksdb/util/channel.h:35:33: error: no matching constructor for initialization of 'std::lock_guard<std::mutex>'

std::lock_guard<std::mutex> lk(lock_);
                            ^  ~~~~~

/usr/include/c++/v1/__mutex_base:90:14: note: candidate constructor not viable: 1st argument ('const std::mutex') would lose const qualifier

explicit lock_guard(mutex_type& __m) _LIBCPP_THREAD_SAFETY_ANNOTATION(acquire_capability(__m))
         ^

/usr/include/c++/v1/__mutex_base:100:5: note: candidate constructor not viable: no known conversion from 'const std::mutex' to 'const std::1::lock_guard<std::1::mutex>' for 1st argument

lock_guard(lock_guard const&) _LIBCPP_EQUAL_DELETE;
^

/usr/include/c++/v1/__mutex_base:94:5: note: candidate constructor not viable: requires 2 arguments, but 1 was provided

lock_guard(mutex_type& __m, adopt_lock_t) _LIBCPP_THREAD_SAFETY_ANNOTATION(requires_capability(__m))
^

1 error generated.

net/ceph* will still not build because Clang crashes but at least
it goes beyond the above error.