cpu_map: Add missing feature "flush-l1d"

Signed-off-by: Tim Wiederhake <twiederh@redhat.com>
Reviewed-by: Michal Privoznik <mprivozn@redhat.com>

tests/cputestdata/x86_64-cpuid-Xeon-Platinum-8268-host.xml

@@ -27,6 +27,7 @@
   <feature name='ospke'/>
   <feature name='md-clear'/>
   <feature name='stibp'/>
+  <feature name='flush-l1d'/>
   <feature name='arch-capabilities'/>
   <feature name='xsaves'/>
   <feature name='mbm_total'/>