rocksdb: enable USE_RTTI=1

This is required for programs using rocksdb and and typeinfo.

Otherwise, linking them fails with errors like this (that's ceph):

/nix/store/cg0k49h66nkdqx6ccwnqr0i4q0fnfznc-binutils-2.31.1/bin/ld: ../../lib/libos.a(RocksDBStore.cc.o):(.data.rel.ro._ZTIN12RocksDBStore14RocksWBHandlerE[_ZTIN12RocksDBStore14RocksWBHandlerE]+0x10): undefined reference to `typeinfo for rocksdb::WriteBatch::Handler'
/nix/store/cg0k49h66nkdqx6ccwnqr0i4q0fnfznc-binutils-2.31.1/bin/ld: ../../lib/libos.a(RocksDBStore.cc.o):(.data.rel.ro._ZTIN12RocksDBStore19MergeOperatorRouterE[_ZTIN12RocksDBStore19MergeOperatorRouterE]+0x10): undefined reference to `typeinfo for rocksdb::AssociativeMergeOperator'
/nix/store/cg0k49h66nkdqx6ccwnqr0i4q0fnfznc-binutils-2.31.1/bin/ld: ../../lib/libos.a(RocksDBStore.cc.o):(.data.rel.ro._ZTIN12RocksDBStore19MergeOperatorLinkerE[_ZTIN12RocksDBStore19MergeOperatorLinkerE]+0x10): undefined reference to `typeinfo for rocksdb::AssociativeMergeOperator'
/nix/store/cg0k49h66nkdqx6ccwnqr0i4q0fnfznc-binutils-2.31.1/bin/ld: ../../lib/libos.a(RocksDBStore.cc.o):(.data.rel.ro._ZTI17CephRocksdbLogger[_ZTI17CephRocksdbLogger]+0x10): undefined reference to `typeinfo for rocksdb::Logger'
/nix/store/cg0k49h66nkdqx6ccwnqr0i4q0fnfznc-binutils-2.31.1/bin/ld: ../../lib/libos.a(BlueRocksEnv.cc.o):(.data.rel.ro._ZTI12BlueRocksEnv[_ZTI12BlueRocksEnv]+0x10): undefined reference to `typeinfo for rocksdb::EnvWrapper'
/nix/store/cg0k49h66nkdqx6ccwnqr0i4q0fnfznc-binutils-2.31.1/bin/ld: ../../lib/libos.a(BlueRocksEnv.cc.o):(.data.rel.ro._ZTI23BlueRocksSequentialFile[_ZTI23BlueRocksSequentialFile]+0x10): undefined reference to `typeinfo for rocksdb::SequentialFile'
/nix/store/cg0k49h66nkdqx6ccwnqr0i4q0fnfznc-binutils-2.31.1/bin/ld: ../../lib/libos.a(BlueRocksEnv.cc.o):(.data.rel.ro._ZTI25BlueRocksRandomAccessFile[_ZTI25BlueRocksRandomAccessFile]+0x10): undefined reference to `typeinfo for rocksdb::RandomAccessFile'
/nix/store/cg0k49h66nkdqx6ccwnqr0i4q0fnfznc-binutils-2.31.1/bin/ld: ../../lib/libos.a(BlueRocksEnv.cc.o):(.data.rel.ro._ZTI21BlueRocksWritableFile[_ZTI21BlueRocksWritableFile]+0x10): undefined reference to `typeinfo for rocksdb::WritableFile'
/nix/store/cg0k49h66nkdqx6ccwnqr0i4q0fnfznc-binutils-2.31.1/bin/ld: ../../lib/libos.a(BlueRocksEnv.cc.o):(.data.rel.ro._ZTI17BlueRocksFileLock[_ZTI17BlueRocksFileLock]+0x10): undefined reference to `typeinfo for rocksdb::FileLock'