Config.in.legacy: fix consistency warning on pthread-stubs option

The old BR2_PACKAGE_PTHREAD_STUBS option (kept for legacy support) now
selects the BR2_PACKAGE_XLIB_LIBPTHREAD_STUBS option, but this creates
a consistency warning from kconfig, because the new option has
BR2_PACKAGE_XORG7 as a dependency.

Since the old option also had BR2_PACKAGE_XORG7 has a dependency, we
make the legacy config option depend on BR2_PACKAGE_XORG7 to avoid
this warning.

Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>
Signed-off-by: Peter Korsgaard <jacmet@sunsite.dk>