xen: ensure xsm-policy is being built when FLASK is enabled

Flask is disabled by default, but this will save someone an
overrideAttrs overlay if they're using FLASK.

Signed-off-by: Fernando Rodrigues <alpha@sigmasquadron.net>

pkgs/applications/virtualization/xen/generic/default.nix

@@ -342,7 +342,9 @@ stdenv.mkDerivation (finalAttrs: {
     ++ lib.lists.optional withInternalOVMF "--enable-ovmf"
 
     ++ lib.lists.optional withIPXE "--with-system-ipxe=${ipxe}"
-    ++ lib.lists.optional withInternalIPXE "--enable-ipxe";
+    ++ lib.lists.optional withInternalIPXE "--enable-ipxe"
+
+    ++ lib.lists.optional withFlask "--enable-xsmpolicy";
 
   makeFlags =
     [