cmake: silence GCC-9 warnings from rocksdb

cmake: silence GCC-9 warnings from rocksdb

GCC-9 complains:

rocksdb/db/version_edit.h:86:8: warning: implicitly-declared ‘constexpr
rocksdb::FileDescriptor::FileDescriptor(const rocksdb::FileDescriptor&)’
is deprecated [-Wdeprecated-copy]
   86 | struct FileMetaData {
      |        ^~~~~~~~~~~~
/var/ssd/ceph/src/rocksdb/db/version_edit.h:55:19: note: because
‘rocksdb::FileDescriptor’ has user-provided ‘rocksdb::FileDescriptor&
rocksdb::FileDescriptor::operator=(const rocksdb::FileDescriptor&)’
   55 |   FileDescriptor& operator=(const FileDescriptor& fd) {
      |                   ^~~~~~~~

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