Fix issue with mkConst/getConst of TypeConstant (#1439)

Fix issue with mkConst/getConst of TypeConstant (#1439)

When compiling the Java bindings on macOS, the linker complained about
CVC4::ExprManager::mkConst<CVC4::TypeConstant>() and
CVC4::Expr::getConst<CVC4::TypeConstant>() being undefined. After some
research, I found that the issue has been introduced by commit
36bf9f8bcb2a1a3aea1f90eb4d13aed3bbf6da8f. It looks like adding the
-no-undefined flags resulted in the symbols in question being omitted
due to TypeConstant not being exported. This commit makes TypeConstant
CVC4_PUBLIC, which fixes the issue.