Eet: fix wrong doxygen definition

Eet: fix wrong doxygen definition

Summary

Eet_Image_Encoding and Eet_Colorclass is from enumeration in Emile,
but it's definition is typedef, not enum.

It makes broken link like below webpage.
https://docs.enlightenment.org/efl/current/group__Eet__File__Image__Group.html

In above page, Eet_Image_Encoding is not linked.

So doxygen documentation should be changed to typedef.

https://phab.enlightenment.org/D5411

Change-Id: I668f178cfda75dd05c794566e8c7a04fe6b769cc
Signed-off-by: JinYong Park <j4939.park@samsung.com>