linking against system library with sqlite_icu on openbsd

[rittneje]

https://github.com/mattn/go-sqlite3/blob/ae2a61f847e10e6dd771ecd4e1c55e0421cdc7f9/sqlite3_opt_icu.go#L15

@gjrtimmer I know this was quite a while ago, but do you remember why this change was added? If you are using the sqlite_icu build tag then you ought to be statically compiling in our version, so linking against the system version seems counter-intuitive. Was this intentional?