session.c: Fix a compiler warning on Mac OS X.

session.c: Fix a compiler warning on Mac OS X.

  session.c:203:22: warning: format specifies type 'unsigned long long'
  but the argument has type 'guint64' (aka 'unsigned long') [-Wformat]
                          sess->session_id, g_variant_get_uint64(data));
  ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~^~~~~~~~~~~~~~~~~~~~~~~~~~
  libsigrokdecode-internal.h:103:44: note: expanded from macro 'srd_dbg'
  #define srd_dbg(...)    srd_log(SRD_LOG_DBG,  __VA_ARGS__)
                                                ^~~~~~~~~~~