epkowa: fix patch download url

the patch was modified (see [1]) so specify the git revision when
downloading it.

[1] https://gitweb.gentoo.org/repo/gentoo.git/commit/media-gfx/iscan/files/iscan-2.28.1.3+libpng-1.5.patch?id=32694b0b3b24bd6729e613fabd7a3050c658c95b