games/springlobby: unbreak with boost 1.70

src/lsl/src/lslutils/misc.cpp:200:58: error: implicit instantiation of undefined template 'boost::shared_ptr<const LSL::CommonUser>'
lslColor GetFreeColor( const ConstCommonUserPtr /*user*/ )

^

src/lsl/src/lslutils/type_forwards.h:15:7: note: template is declared here
class shared_ptr;

^

Obtained from: https://github.com/springlobby/lsl/commit/5134cf61436b