libjcat: support cross compilation

gnutls' certtool binary is required by libjcat during build (`data/tests/meson.build`),
and gnutls is linked into the libjcat.so target,
hence it appears in both nativeBuildInputs and buildInputs.

pkgs/development/libraries/libjcat/default.nix

@@ -43,6 +43,7 @@ stdenv.mkDerivation rec {
     docbook-xsl-nons
     gobject-introspection
     vala
+    gnutls
     gtk-doc
     python3
   ];