FreeBSD shows a combination of define cases that make the function
missing_encryption_key unused. Put it in the right define scope.
Change-Id: Icf226f134ffa7624a7daf709c302c29acfffe95e
Reviewed-on: https://code.wireshark.org/review/37360
Petri-Dish: Guy Harris <gharris@sonic.net>
Tested-by: Petri Dish Buildbot
Reviewed-by: Guy Harris <gharris@sonic.net>