gaw: mark as Linux-only

The error when building on Darwin is:

sockcon.c:19:10: fatal error: 'linux/tcp.h' file not found

So, it uses Linux-specific headers.

pkgs/applications/science/electronics/gaw/default.nix

···
   buildInputs = [ gtk3 ];
 
   meta = with lib; {
-    broken = stdenv.isDarwin;
     description = "Gtk Analog Wave viewer";
     longDescription = ''
       Gaw is a software tool for displaying analog waveforms from
···
     homepage = "http://gaw.tuxfamily.org";
     license = licenses.gpl2Plus;
     maintainers = with maintainers; [ fbeffa ];
-    platforms = platforms.all;
+    platforms = platforms.linux;
   };
 }