unifdef: set platforms to unix, not linux

Apparently, Darwin evaluation is somehow busted without this, although
I can't verify it. Reported by @matthewbauer on IRC. I'll take the
blame if this somehow makes life worse.

/cc @vrthra as maintainer

Signed-off-by: Austin Seipp <[e-mail]>

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

···
     homepage = "http://dotat.at/prog/unifdef/";
     description = "Selectively remove C preprocessor conditionals";
     license = licenses.bsd2;
-    platforms = platforms.linux;
+    platforms = platforms.unix;
     maintainers = [ maintainers.vrthra ];
   };
 }