sstate.bbclass: update .siginfo atime

sstate.bbclass: update .siginfo atime

.siginfo files are not being accessed from local or NFS-mounted
sstate mirrors when sstate package is installed, so their atime
is not updated. If sstate mirror is cleaned based on access time,
they get deleted, even though they are still being used.

Updated atime of .siginfo symlinks with 'touch -a'. This command
dereferences symlinks pointing to the local mirror and updates
atime of the .siginfo file on the mirror.

[YOCTO #10857]

(From OE-Core rev: fb1499a42756faeef025122bbde98bc14f4ae61e)

Signed-off-by: Ed Bartosh <ed.bartosh@linux.intel.com>
Signed-off-by: Ross Burton <ross.burton@intel.com>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>