meme-image-generator: add mainProgram

Signed-off-by: Christina Sørensen <[e-mail]>

pkgs/applications/graphics/meme-image-generator/default.nix

···
     homepage = "https://github.com/nomad-software/meme";
     license = licenses.mit;
     maintainers = [ maintainers.fgaz ];
+    mainProgram = "meme";
   };
 }