Add fix for compile flycheck problem

user/emacs.nix

@@ -1251,6 +1251,12 @@ in
         hook = [ "ggtags-mode" ];
       };
 
+      # Needed by Flycheck.
+      pkg-info = {
+        enable = true;
+        command = [ "pkg-info-version-info" ];
+      };
+
       protobuf-mode = {
         mode = [ ''"'\\.proto\\'"'' ];
       };