Quell compiler warning.

Building on a 32-bit linux system was generating the following:

  src/st-info.c: In function ‘print_data’:
  src/st-info.c:25:3: warning: format ‘%lx’ expects argument of type \
       ‘long unsigned int’, but argument 2 has type ‘size_t’ [-Wformat]
  src/st-info.c:27:3: warning: format ‘%lx’ expects argument of type \
       ‘long unsigned int’, but argument 2 has type ‘size_t’ [-Wformat]
  src/st-info.c:29:3: warning: format ‘%lx’ expects argument of type \
       ‘long unsigned int’, but argument 2 has type ‘size_t’ [-Wformat]

Using '%zx' eliminates the warning in a platform agnostic way.