Export sqlite3_index_constraint, sqlite3_index_orderby, and sqlite3_index_constraint_usage structs in lib

I'm working on implementing user-defined virtual tables in my package (zombiezen/go-sqlite#15). In order to implement the xBestIndex callback, I need access to the sqlite3_index_constraint, sqlite3_index_orderby, and sqlite3_index_constraint_usage structs, which are present in modernc.org/sqlite/lib, but not exported. IIUC it's because they declared inline but not typedef'd.

I'm not sure the best way to do this with the modernc.org compiler, else I would send this as a PR. Thanks as always!