package/firewalld: add missing nftables json runtime dependency

Now that firewalld has been fixed for Python 3.13,
TestFirewalldSysVInit still fail at runtime due to missing nftables json
support [1].

cat /var/log/firewalld
  2025-09-02 21:34:50 Traceback (most recent call last):
    File "/usr/lib/python3.13/site-packages/firewall/core/fw.py", line 649, in start
    File "/usr/lib/python3.13/site-packages/firewall/core/fw.py", line 613, in _start
    File "/usr/lib/python3.13/site-packages/firewall/core/fw.py", line 510, in _start_apply_objects
    File "/usr/lib/python3.13/site-packages/firewall/core/fw_transaction.py", line 133, in execute
  firewall.errors.FirewallError: COMMAND_FAILED: 'python-nftables' failed: Error: syntax error, unexpected '{'
  {"nftables": [{"metainfo": {"json_schema_version": 1}}, {"add": {"table": {"family": "inet", "name": "firewalld"}}}, {"delete": {"table": {"family": "inet", "name": "firewalld"}}}, {"add": {"table": {"family": "inet", "name": "firewalld"}}}]}
  ^

Fixes:
https://gitlab.com/buildroot.org/buildroot/-/jobs/11198422467 (TestFirewalldSystemd)

[1] https://github.com/firewalld/firewalld/commit/1582c5dd736a11177ec320eb4ecbeb09fcae313a

Signed-off-by: Romain Naour <romain.naour@smile.fr>
Signed-off-by: Julien Olivain <ju.o@free.fr>

package/firewalld/Config.in

@@ -14,6 +14,7 @@ config BR2_PACKAGE_FIREWALLD
 	select BR2_PACKAGE_GOBJECT_INTROSPECTION
 	select BR2_PACKAGE_JANSSON # Uses the nftables json interface
 	select BR2_PACKAGE_NFTABLES
+	select BR2_PACKAGE_NFTABLES_JSON # runtime
 	select BR2_PACKAGE_PYTHON3
 	select BR2_PACKAGE_PYTHON_GOBJECT
 	help