games/openttd: fix build with LLVM 15 on powerpc*

Use LLVM 16 instead:
Assertion failed: (!KnownValid && "Explicit template arguments?"), function AddOverloadedCallCandidate, file /usr/local/poudriere/jails/main-powerpc64le/usr/src/contrib/llvm-project/clang/lib/Sema/SemaOverload.cpp, line 12672.