games/pokerth: Fix build with protobuf 29+

ld: error: undefined symbol: absl::lts_20240722::log_internal::kCharNull
>>> referenced by clientthread.cpp
>>>               clientthread.o:(std::__1::basic_string<char, std::__1::char_traits<char>, std::__1::allocator<char>>* absl::lts_20240722::log_internal::MakeCheckOpString<char const*, void const*>(char const*, void const*, char const*)) in archive lib/libpokerth_lib.a
c++: error: linker command failed with exit code 1 (use -v to see invocation)

Approved by:	portmgr (blanket)