x11-toolkits/p5-Alien-wxWidgets: add missing file to pkg-plist

This appears to have been overlooked in a previous update, so note its
presence in the pkg-plist.

This fixes Q/A checks run by poudriere testport.

Approved by: koobs (mentor), bapt (mentor)
MFH: 2020Q1 (packaging fix)
Differential Revision: D23702 (included, but not the primary change)