layer.conf: Consider shared-mime-info and desktop-file-utils siggen safe
These are added as dependencies by mime class, which could be inherited
along with allarch and dependencies silently go into target specific
recipes and which then can result in sstate hash signature mismatches
when the allarch recipes are reused for another arch
[YOCTO #13805]
Signed-off-by: Khem Raj <raj.khem@gmail.com> Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>
