NCP: add a missing cast following g6ca40c5

Change-Id: I0b93c4faab52d830b8160d89b9bb8026ab2a216e
Reviewed-on: https://code.wireshark.org/review/4657
Reviewed-by: Pascal Quantin <pascal.quantin@gmail.com>
(cherry picked from commit 63faad686b49e02b86cd1961dd3bc8283f120da8)
Reviewed-on: https://code.wireshark.org/review/4660