Block ReadOnlyDB in ROCKSDB_LITE

Summary:
db_imp_readonly.o is one of the big obj file.  If it's not a necessary
feature, we should probably block it in ROCKSDB_LITE.

    1322704 Nov 24 16:55 db/db_impl_readonly.o

Test Plan:
make shared_lib -j32
make ROCKSDB_LITE shared_lib -j32

Reviewers: ljin, igor

Reviewed By: igor

Subscribers: dhruba, leveldb

Differential Revision: https://reviews.facebook.net/D29583