texi2html: change platforms to unix

Tested on Darwin

pkgs/development/tools/misc/texi2html/default.nix

···
     homepage = http://www.nongnu.org/texi2html/;
     license = stdenv.lib.licenses.gpl2;
     maintainers = [stdenv.lib.maintainers.marcweber];
-    platforms = stdenv.lib.platforms.linux;
+    platforms = stdenv.lib.platforms.unix;
   };
 }