dns320l-mcu: Fix compilation with GCC 14

dns320l-mcu: Fix compilation with GCC 14

This fixes the following compile problem:
```
dns320l-daemon.c: In function 'main':
dns320l-daemon.c:740:18: error: implicit declaration of function 'isprint' [-Wimplicit-function-declaration]
  740 |         else if (isprint (optopt))
      |                  ^~~~~~~
dns320l-daemon.c:50:1: note: include '<ctype.h>' or provide a declaration of 'isprint'
   49 | #include "dns320l-daemon.h"
  +++ |+#include <ctype.h>
   50 |
dns320l-daemon.c:799:5: error: implicit declaration of function 'umask' [-Wimplicit-function-declaration]
  799 |     umask(0);
      |     ^~~~~
dns320l-daemon.c:864:5: error: 'return' with no value, in function returning non-void [-Wreturn-mismatch]
  864 |     return;
      |     ^~~~~~
dns320l-daemon.c:691:5: note: declared here
  691 | int main(int argc, char *argv[])
      |     ^~~~
```

Link: https://github.com/openwrt/openwrt/pull/18688
Signed-off-by: Hauke Mehrtens <[email protected]>