Fix crash when an application wants to get the popup item class

Fix crash when an application wants to get the popup item class

Since elm_popup_item.eo.h is only included in elc_popup.h, the EAPI is
not well defined, resulting in a crash when, in a simple C file, the
item class is tried to be accessed.
By including the H file in elc_popup_eo.h, we make it public (as the
other item classes) and solve the EAPI issue.

Thanks @jpeg for helping me to solve this issue.