yosys/plugins/ghdl.nix: fix build

Since 12cc34ce2dd2f all yosys plugins are expected to have a
`plugin` attribute; without it they will not build. This commit
adds one for yosys-ghdl.

pkgs/development/compilers/yosys/plugins/ghdl.nix

···
   pname = "yosys-ghdl";
   # This is not the latest commit, but it's the latest that builds with current stable ghdl 2.0.0
   version = "2022.01.11";
+  plugin = "ghdl";
 
   src = fetchFromGitHub {
     owner  = "ghdl";