Fix global plugin folder when it is set as an absolute path

This fixes regression in creating Debian packages caused by
80b36cdbde90ac9639938a14e8fd90a79602666a

Change-Id: I46702ad4c9a11ce317da7ef664c84e1f22d3f709
Reviewed-on: https://code.wireshark.org/review/5316
Reviewed-by: Evan Huus <eapache@gmail.com>
Petri-Dish: Evan Huus <eapache@gmail.com>
Reviewed-by: Balint Reczey <balint@balintreczey.hu>