MFH: r430857

irc/quassel: Fix erroneously empty value for PLIST variable

This was causing the build of slave port irc/quassel-core to hang during
the ports build step which generates temporary packing list, as the sed
command to perform PLIST_SUB did not have a filename argument provided.

Reported by: Steve Hay <hay.steve@gmail.com>

Approved by: ports-secteam (junovitch)