uhexen2: fix the build against SDL_compat

Without the change build fails due to missing libraries as
https://hydra.nixos.org/build/294936344:

../h2shared/glheader.h:40:10: fatal error: GL/gl.h: No such file or directory
40 | #include <GL/gl.h>
| ^~~~~~~~~

+4
+4
pkgs/by-name/uh/uhexen2/package.nix
··· 3 fetchgit, 4 SDL, 5 stdenv, 6 libogg, 7 libvorbis, 8 libmad, ··· 21 22 buildInputs = [ 23 SDL 24 libogg 25 libvorbis 26 libmad
··· 3 fetchgit, 4 SDL, 5 stdenv, 6 + alsa-lib, 7 + libGL, 8 libogg, 9 libvorbis, 10 libmad, ··· 23 24 buildInputs = [ 25 SDL 26 + alsa-lib 27 + libGL 28 libogg 29 libvorbis 30 libmad