beep: set meta.platforms

beep includes <linux/xyz.h> C header files, so I'm setting platforms to
Linux only.

pkgs/misc/beep/default.nix

@@ -20,5 +20,6 @@ stdenv.mkDerivation {
     description = "The advanced PC speaker beeper";
     homepage = http://www.johnath.com/beep/;
     license = stdenv.lib.licenses.gpl2;
+    platforms = stdenv.lib.platforms.linux;
   };
 }