sdcc: clarify license

Ref https://github.com/jtojnar/nixpkgs-hammering/blob/master/explanations/unclear-gpl.md

pkgs/development/compilers/sdcc/default.nix

···
       PIC18 targets. It can be retargeted for other microprocessors.
     '';
     homepage = "http://sdcc.sourceforge.net/";
-    license = with licenses; if (gputils == null) then gpl2 else unfreeRedistributable;
+    license = with licenses; if (gputils == null) then gpl2Plus else unfreeRedistributable;
     maintainers = with maintainers; [ bjornfor yorickvp ];
     platforms = platforms.all;
   };