Fixing compiler problem of main.

[AnsgarSchmidt]

When I compile the code, I get this error. The patch fixes this. Very simple, only include.

cc -Wall -Wextra -g   -o rpiboot main.c bootfiles.c decode_duid.c `pkg-config --cflags --libs libusb-1.0` -DGIT_VER="\"57c61e97\"" -DPKG_VER="\"local\"" -DBUILD_DATE="\"2025/12/11\"" -DDEFAULT_MSG_DIR=\"/usr/share/rpiboot/mass-storage-gadget64/\" 
main.c: In function ‘get_options’:
main.c:558:46: warning: implicit declaration of function ‘isdigit’ [-Wimplicit-function-declaration]
  558 |                 else if((*argv)[0] == '-' && isdigit((unsigned char)(*argv)[1]))
      |                                              ^~~~~~~
main.c:13:1: note: include ‘<ctype.h>’ or provide a declaration of ‘isdigit’
   12 | #include "msd/bootcode4.h"
  +++ |+#include <ctype.h>
   13 | // 2712 doesn't use start5.elf

[timg236]

Please can you fix this to be a linear commit rather than a merge