mailutils: fix build with gcc15

- add patch from archlinux fixing `incompatible-pointer-types` error:
https://gitlab.archlinux.org/archlinux/packaging/packages/mailutils/-/raw/87c3614083260f52dd1222e872a1836f0ff9abe1/fix-build.patch

Upstream issues:
https://lists.gnu.org/archive/html/bug-mailutils/2025-06/msg00000.html
https://lists.gnu.org/archive/html/bug-mailutils/2025-09/msg00000.html

Fixes build failure with gcc15:
```
progmailer.c: In function 'mu_progmailer_create':
progmailer.c:63:18: error: assignment to 'void (*)(void)' from
incompatible pointer type 'void (*)(int)' [-Wincompatible-pointer-types]
   63 |   pm->sighandler = SIG_ERR;
      |                  ^
progmailer.c: In function 'mu_progmailer_open':
progmailer.c:116:23: error: assignment to 'void (*)(void)' from
incompatible pointer type '__sighandler_t' {aka 'void (*)(int)'} [-Wincompatible-pointer-types]
  116 |   if ((pm->sighandler = signal (SIGCHLD, SIG_DFL)) == SIG_ERR)
      |                       ^
In file included from progmailer.c:27:
/nix/store/i1qha7him4faq593wwl1zmhg2pc6lz98-glibc-2.40-66-dev/include/signal.h:72:16:
note: '__sighandler_t' declared here
   72 | typedef void (*__sighandler_t) (int);
      |                ^~~~~~~~~~~~~~
progmailer.c:116:52: warning: comparison of distinct pointer types
lacks a cast [-Wcompare-distinct-pointer-types]
  116 |   if ((pm->sighandler = signal (SIGCHLD, SIG_DFL)) == SIG_ERR)
      |                                                    ^~
progmailer.c: In function 'mu_progmailer_close':
progmailer.c:295:22: warning: comparison of distinct pointer types
lacks a cast [-Wcompare-distinct-pointer-types]
  295 |   if (pm->sighandler != SIG_ERR
      |                      ^~
progmailer.c:296:29: error: passing argument 2 of 'signal' from
incompatible pointer type [-Wincompatible-pointer-types]
  296 |       && signal (SIGCHLD, pm->sighandler) == SIG_ERR)
      |                           ~~^~~~~~~~~~~~
      |                             |
      |                             void (*)(void)
/nix/store/i1qha7him4faq593wwl1zmhg2pc6lz98-glibc-2.40-66-dev/include/signal.h:88:57:
note: expected '__sighandler_t' {aka 'void (*)(int)'} but argument is of type 'void (*)(void)'
   88 | extern __sighandler_t signal (int __sig, __sighandler_t __handler)
      |                                          ~~~~~~~~~~~~~~~^~~~~~~~~
/nix/store/i1qha7him4faq593wwl1zmhg2pc6lz98-glibc-2.40-66-dev/include/signal.h:72:16:
note: '__sighandler_t' declared here  
   72 | typedef void (*__sighandler_t) (int);
      |                ^~~~~~~~~~~~~~
progmailer.c:302:18: error: assignment to 'void (*)(void)' from
incompatible pointer type 'void (*)(int)' [-Wincompatible-pointer-types]
  302 |   pm->sighandler = SIG_ERR;
      |                  ^
```

0 files changed, 0 insertions, 0 deletions