packagegroup-fslc-gstreamer1.0: set PACKAGE_ARCH before packagegroup inherit

The required order was changed in OE-Core commit:

,----
| allarch: Add warning about packagegroup
|
| Since we want to start including this class conditionally, detect cases
| where packagegroup files use the old ordering and inform the user they
| need to update this.
`----

This was a left over in last commit, fixing the other instances of
same issue.

Change-Id: I75895c40667507dc7446a579e2d9284c688c725c
Signed-off-by: Otavio Salvador <otavio@ossystems.com.br>