python3: fix PGO for non-reproducible biniaries

When PGO was disabled by default by e53ebf29, a bug was introduced that
prevented PGO from ever being enabled. At the time at which extra config
is appended to PACKAGECONFIG_CONFARGS, PACKAGECONFIG_PGO remains
unevaluated in PACKAGECONFIG_class-target, due to setting its value in
an anonymous Python function. As a result, the PGO options options will
never be included.

Signed-off-by: Ryan Rowe <rrowe@xevo.com>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>