imagemagick: use download location as hash source

Since commit 12a6c5b1 (imagemagick: use official download site,
2015-02-17) we use the official ImageMagic size. Use it also as hash source.
That's the conversion we use in all other packages when upstream provides
hashes.

Cc: Fabio Porcedda <Fabio.Porcedda@gmail.com>
Signed-off-by: Baruch Siach <baruch@tkos.co.il>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>