Add Wireshark_LIBRARY variable

This variable can be used to obtain the wireshark library(ies) (from the correct location).

Change-Id: I90cd5b098eb746ac573a2c611113287b06c3b82b
Reviewed-on: https://code.wireshark.org/review/5251
Petri-Dish: Michael Mann <mmann78@netscape.net>
Tested-by: Petri Dish Buildbot <buildbot-no-reply@wireshark.org>
Reviewed-by: Michael Mann <mmann78@netscape.net>