#9 Fix building with GCC 14

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Newer compiler like GCC 14 and Clang 16 (and greater) enable a few
compiler options by default that result in build error,
-Wincompatible-pointer-types being one of them. Thus giving us

dga.c:392:32: error: assignment to 'ggi_modelist_validate' {aka 'int (*)(ggi_visual *, long int,  ggi_mode *)'} from incompatible pointer type 'int (*)(ggi_visual *, int,  ggi_mode *)' [-Wincompatible-pointer-types]
  392 |         priv->mlfuncs.validate = ggi_xdga_validate_mode;
      |                                ^

First reported on Gentoo linux at https://bugs.gentoo.org/930101.