Wrong formatting verb in `main.c`

[phillipstanleymarbell]

main.c:781:34: warning: format ‘%d’ expects argument of type ‘int’, but argument 2 has type ‘size_t {aka long unsigned int}’ [-Wformat=]
   printf("Unknown memory length %d in mmap file.\n", strlen(p));
                                 ~^                   ~~~~~~~~~
                                 %ld