Fix duplicate symbols in downstream crates (#95)

Replace `const` with `static const` for internal linkage.

> rust-lld: error: duplicate symbol: RS_UA_STATUSCODE_BADOUTOFMEMORY
> defined at wrapper.h:32
(/home/runner/.cargo/git/checkouts/open62541-sys-f44ba0cbd54aa9ea/dd4b0e2/wrapper.h:32)

Author: uklotzde