target-sdk-provides-dummy: resolve sstate conflict

It exists a situation that there is a common config file includes
multilib.conf but variable MULTILIBS is not set by default:

  require conf/multilib.conf
  MULTILIBS ?= ""

When build target-sdk-provides-dummy in a build project, it fails with
following steps:

1 $ echo 'MACHINE = "qemux86"' >>conf/local.conf
  $ bitbake target-sdk-provides-dummy
2 $ cat <<EOF >>conf/local.conf
    MACHINE = "qemux86-64"
    MULTILIBS = "multilib:lib32"
    DEFAULTTUNE_virtclass-multilib-lib32 = "i586"
    EOF
  $ bitbake target-sdk-provides-dummy
  $ bitbake lib32-target-sdk-provides-dummy

It fails to build lib32-target-sdk-provides-dummy with error messages:

| ERROR: target-sdk-provides-dummy-1.0-r0 do_packagedata: The recipe target-sdk-provides-dummy
|  is trying to install files into a shared area when those files already exist. Those files
|  and their manifest location are:
|   .../tmp/pkgdata/qemux86-64/lib32-target-sdk-provides-dummy
|     (matched in manifest-qemux86_64-lib32-target-sdk-provides-dummy.packagedata)
|   .../tmp/pkgdata/qemux86-64/runtime/lib32-target-sdk-provides-dummy
|     (matched in manifest-qemux86_64-lib32-target-sdk-provides-dummy.packagedata)
|   ... snip ...
| Please verify which recipe should provide the above files.

Add related directories to SSTATE_DUPWHITELIST to avoid the failures.

Signed-off-by: Kai Kang <kai.kang@windriver.com>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>