Fix "known incorrect sRGB profil" in icons/staged-item.png

icons/staged-item.png has a broken profile which results in this icon not
shown when used with latest libpng. See following thread for extensive
information (http://comments.gmane.org/gmane.linux.gentoo.devel/85066).

Signed-off-by: Justin Lecher <jlec@gentoo.org>