sstate: use only unique set of SSTATETASK

sstate: use only unique set of SSTATETASK

* otherwise strange error like this:
  ERROR: Logfile of failure stored in: /OE/shr-core/tmp/work/armv7a-oe-linux-gnueabi/libtool-cross-2.4-r1/temp/log.do_package_write_ipk.25551
  Log data follows:
  | ERROR: Package already staged (/OE/shr-core/tmp/sstate-control/manifest-nokia900-libtool-cross.deploy-ipk)?!
  | ERROR: Function 'sstate_task_postfunc' failed
  NOTE: package libtool-cross-2.4-r1: task do_package_write_ipk: Failed
  ERROR: Task 11 (/OE/shr-core/openembedded-core/meta/recipes-devtools/libtool/libtool-cross_2.4.bb, do_package_write_ipk) failed with exit code '1'

  is shown in this case with package_ipk twice in INHERIT

* Thanks to Richard for fix

Signed-off-by: Martin Jansa <Martin.Jansa@gmail.com>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>