warning: token pasting of ',' and __VA_ARGS__ is a GNU extension

Comes up w/ recent Clang:
```
libxcf/xcf.c:49:67: warning: token pasting of ',' and __VA_ARGS__ is a GNU extension [-Wgnu-zero-variadic-macro-arguments]
   49 | #define PRINT_ERROR(msg, ...) fprintf(stderr, "[libxcf] " msg "\n", ##__VA_ARGS__)
      |                                                                   ^
```