Fix compiler warnings: invalid type format

This patch fixes the following warnings:

clients/weston-info.c: In function 'print_tablet_tool_info':
clients/weston-info.c:569:3: warning: format '%lx' expects argument of type 'long unsigned int', but argument 2 has type 'uint64_t' [-Wformat=]
   printf("\t\t\thardware serial: %lx\n", info->hardware_serial);
   ^
clients/weston-info.c:572:3: warning: format '%lx' expects argument of type 'long unsigned int', but argument 2 has type 'uint64_t' [-Wformat=]
   printf("\t\t\thardware wacom: %lx\n", info->hardware_id_wacom);

Signed-off-by: Marius Vlad <marius.vlad0@gmail.com>