colorspaces: adobergb gamma is technically not quite 2.2

from: https://github.com/hughsie/colord/commit/7efe9fe132dac72adf06c5a36112b8028d0499b4

According to http://www.adobe.com/digitalimag/pdfs/AdobeRGB1998.pdf they are
*not* 2.2, but '2 + 51/256' which isn't quite 2.2.

This is also the value used by ArgyllCMS, many thanks to Elle Stone for the hint.

Important practical note is that in both cases (2.2 and 2.19921875) results in a profile
encoded with gamma 2.199219 which is probably due to limited accuracy in the ICC spec.