shortcuts: make mavpicviewer.desktop executable like the other two

It was the only launcher at mode 100644 while mavproxy.desktop and
mavexplorer.desktop are 100755. The installer chmods what it copies, so
this only matters for the documented manual route of copying a .desktop
file to the Desktop, where GNOME requires the executable bit.

Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>

Author: 2 people