classes/buildhistory: fix for sstate class change

SSTATEPOSTINSTFUNCS is now set with = in sstate.bbclass, and because
the line here in buildhistory.bbclass ends up being parsed before that
we now need to use _append or the value we are adding will be wiped out.
This fixes buildhistory no longer recording package information since
OE-Core revision 9d659c6f20fa4a141b491c62a3ef0dfb1f896d9c.

Signed-off-by: Paul Eggleton <paul.eggleton@linux.intel.com>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>