Force linking with pthread under unix

Protobuf depends on pthread under unix, but cmake’s FindProtobuf
doesn’t add -lpthread to link flags

common/pb/CMakeLists.txt

@@ -157,4 +157,7 @@ set(cockatrice_protocol_LIBS ${PROTOBUF_LIBRARIES})
 if (MSVC)
     set(cockatrice_protocol_LIBS ${cockatrice_protocol_LIBS} -lprotobuf)
 endif (MSVC)
+if (UNIX)
+    set(cockatrice_protocol_LIBS ${cockatrice_protocol_LIBS} -lpthread)
+endif (UNIX)
 target_link_libraries(cockatrice_protocol ${cockatrice_protocol_LIBS})