Explicitly set RGB colourspace in icon.pl's use of convert.

Explicitly set RGB colourspace in icon.pl's use of convert.

This is that annoying feature of up-to-date 'convert' in which
converting to or from a PNG file defaults to returning RGB values that
have been 'helpfully' gamma-corrected (or some such) from the exact
data stored in the source file to some nonsense you didn't want.

Usually the worst this causes is slightly washed-out looking graphics,
but in this case, since my entire aim was to squash the image into a
specific set of exact RGB values so as to turn it into a paletted
Windows icon file, it caused an actual build failure when the next
loop in icon.pl couldn't find the gamma-corrected values in its
expected palette map, and no wonder.