Add deps/assets/igmt.ico -- it was never committed, only local

Every reference to deps/assets/igmt.ico this session (CPack packaging,
make_desktop_shortcut.vbs) worked here because the file physically
existed on this dev machine, but it was untracked the whole time -- a
fresh Pkg.add/Pkg.update checkout on another machine genuinely didn't
have it, so make_desktop_shortcut.vbs's CopyFile failed with "File not
found" trying to read it as a source. Should have caught this from the
very first git status of the session; didn't.

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

Author: joa-quim