c/r: use PRIu64 format specifier

c/r: use PRIu64 format specifier

Fixes build failures on arm:

criu.c: In function ‘exec_criu’:
criu.c:310:4: error: format ‘%lu’ expects argument of type ‘long unsigned int’, but argument 3 has type ‘uint64_t’ [-Werror=format=]
    ret = sprintf(ghost_limit, "%lu", opts->user->ghost_limit);
    ^
In file included from criu.c:42:0:
log.h:285:9: error: format ‘%lu’ expects argument of type ‘long unsigned int’, but argument 3 has type ‘uint64_t’ [-Werror=format=]
  struct lxc_log_locinfo locinfo = LXC_LOG_LOCINFO_INIT;  \
         ^
criu.c:312:5: note: in expansion of macro ‘ERROR’
     ERROR("failed to print ghost limit %lu", opts->user->ghost_limit);
     ^

Signed-off-by: Christian Brauner <cbrauner@suse.de>