Fix plugin search path: It was relative instead of absolute, resulting
in not found plugins (lib/wireshark/plugins/1.99.1).

Change-Id: Ie81f3c4450789ff7e37c99e197b44e7df47d8df5
Reviewed-on: https://code.wireshark.org/review/5149
Reviewed-by: Jörg Mayer <jmayer@loplof.de>