cups-filters: fix build with gcc15

- add patch from merged upstream PR:
https://www.github.com/OpenPrinting/cups-filters/pull/618
https://github.com/OpenPrinting/cups-filters/commit/9871a50b5c1f9c2caa2754aac1f5db70c886021b

Fixes build failure with gcc15:
```
filter/foomatic-rip/renderer.c: In function 'exec_kid3':
filter/foomatic-rip/renderer.c:456:32: error: passing argument 2 of
'start_process' from incompatible pointer type [-Wincompatible-pointer-types]
  456 |   kid4 = start_process("kid4", exec_kid4, NULL, &kid4in, NULL);
      |                                ^~~~~~~~~
      |                                |
      |                                int (*)(FILE *, FILE *, void *)
In file included from filter/foomatic-rip/renderer.c:21:
filter/foomatic-rip/process.h:21:45: note: expected 'int (*)(void)' but
argument is of type 'int (*)(FILE *, FILE *, void *)'
   21 | pid_t start_process(const char *name, int (*proc_func)(), void *user_arg,
      |                                       ~~~~~~^~~~~~~~~~~~
filter/foomatic-rip/renderer.c:384:1: note: 'exec_kid4' declared here
  384 | exec_kid4(FILE *in,
      | ^~~~~~~~~
filter/foomatic-rip/process.c:231:1: error: conflicting types for 'start_process';
have 'pid_t(const char *, int (*)(FILE *, FILE *, void *), void *, FILE **, FILE **)'
{aka 'int(const char *, int (*)(FILE *, FILE *, void *), void *, FILE **, FILE **)'}
  231 | start_process(const char *name,
      | ^~~~~~~~~~~~~
In file included from filter/foomatic-rip/process.c:14:
filter/foomatic-rip/process.h:21:7: note: previous declaration of 'start_process'
with type 'pid_t(const char *, int (*)(void), void *, FILE **, FILE **)'
{aka 'int(const char *, int (*)(void), void *, FILE **, FILE **)'}
   21 | pid_t start_process(const char *name, int (*proc_func)(), void *user_arg,
      |       ^~~~~~~~~~~~~
```