krita_qimageio.desktop: Fix MimeType separators

Summary:
desktop file did not pass validation.

/usr/share/applications/krita_qimageio.desktop: error: (will be fatal in the future): value "image/bmp,image/x-xpixmap,image/x-xbitmap,image/webp,image/vnd.microsoft.icon" for key
"MimeType" in group "Desktop Entry" contains value "image/bmp,image/x-xpixmap,image/x-xbitmap,image/webp,image/vnd.microsoft.icon" which is an invalid MIME type:
"image/bmp,image/x-xpixmap,image/x-xbitmap,image/webp,image/vnd.microsoft.icon" contains an invalid character in the subtype

Reviewers: Krita, rempt

Reviewed By: Krita, rempt

Tags: Krita

Differential Revision: https://phabricator.kde.org/D16222

(cherry picked from commit d267979af9524372f00a0759f02e75c60f604da5)