gpgme: fix the qt5 bindings

Commit 1340607f3d (PR #155384) broke list-nesting of the passed qtbase.

Surprisingly to me, that prevented it from appearing in PKG_CONFIG_PATH
and thus from building the Qt bindings.  Subsequently those were then
missing in libsForQt5.libkleo and maybe other places.

pkgs/development/libraries/gpgme/default.nix

@@ -71,7 +71,7 @@ stdenv.mkDerivation rec {
     libassuan
     libgpg-error
     pth
-  ] ++ lib.optional (qtbase != null) [
+  ] ++ lib.optionals (qtbase != null) [
     qtbase
   ];