fix: make icebox.py executable

[Xiretza]

f4pga-arch-defs expects to find an executable called icebox.py on the PATH, and while the file exists, it is not marked +x, so configuration fails.

[lethalbit]

It might be better to ensure the install step marks the file as executable as opposed to setting +x directly on the file itself, as there is potential for the executable flag to be cleared if, for instance, it is first moved onto an NTFS file system.

So it might be better to add the executable bit to the install step in the Makefile