Fix typo in printf format for session.cpp

Fixes:
bindings/cpp/session.cpp:187:60: error: format '%ld' expects argument of type 'long int', but argument 4 has type 'uint64_t {aka long long unsigned int}' [-Werror=format]
| cc1plus: all warnings being treated as errors
| make[2]: *** [bindings/cpp/CMakeFiles/elliptics_cpp.dir/session.cpp.o] Error 1

Signed-off-by: Samuel Stirtzel <s.stirtzel@googlemail.com>