edac-utils: set meta.mainProgram

There is also `edac-ctl`, but this seems more likely what one wants
*most* of the time. Either way, better than no mainProgram.

pkgs/by-name/ed/edac-utils/package.nix

···
   meta = with lib; {
     homepage = "https://github.com/grondo/edac-utils";
     description = "Handles the reporting of hardware-related memory errors";
+    mainProgram = "edac-util";
     license = licenses.gpl2Plus;
     platforms = platforms.linux;
   };