unifdef: fix build with gcc15

- add patch from unmerged upstream PR:
https://www.github.com/fanf2/unifdef/pull/19

Fixes build failure with gcc15:
```
unifdef.c:205:1: error: 'constexpr' in empty declaration
unifdef.c: In function 'eval_unary':
unifdef.c:1089:27: error: expected identifier or '(' before '=' token
 1089 |                 constexpr = false;
      |                           ^
unifdef.c:1106:27: error: expected identifier or '(' before '=' token
 1106 |                 constexpr = false;
      |                           ^
unifdef.c: In function 'ifeval':
unifdef.c:1173:19: error: expected identifier or '(' before '=' token
 1173 |         constexpr = killconsts ? false : true;
      |                   ^
unifdef.c:1176:27: error: expected specifier-qualifier-list before '?' token
 1176 |         return (constexpr ? LT_IF : ret == LT_ERROR ? LT_IF : ret);
      |                           ^
```

0 files changed, 0 insertions, 0 deletions