nitg: fix FFI error with arguments name of extern methods

Signed-off-by: Alexis Laferrière <alexis.laf@xymus.net>
nitg-s: activate FFI in nitg-s (but not nitg-e)

Signed-off-by: Alexis Laferrière <alexis.laf@xymus.net>
tests: skip test_json in nitg-s

Signed-off-by: Alexis Laferrière <alexis.laf@xymus.net>
nitg-s: fix "No provided declaration" with FFI casts

Signed-off-by: Alexis Laferrière <alexis.laf@xymus.net>