ftmod_misdn: Fix %lx vs.__u64 format string error

Fixes:
      src/ftmod/ftmod_misdn/ftmod_misdn.c:924:3: error: format '%lx' expects
      argument of type 'long unsigned int', but argument 13 has type '__u64'
      [-Werror=format]

Signed-off-by: Stefan Knoblich <stkn@openisdn.net>
Reported-by: hyper_ch #freetdm @ irc.freenode.net