of: fix unittest build without CONFIG_OF_OVERLAY

We get a link error when the new tests are used by overlays
are not:

drivers/of/built-in.o: In function `unflatten_device_tree':
(.init.text+0x967): undefined reference to `unittest_unflatten_overlay_base'

This makes the #ifdef check match the symbols that lead to building
the unittest_unflatten_overlay_base function.

Fixes: 81d0848f ("of: Add unit tests for applying overlays")
Signed-off-by: Arnd Bergmann <arnd@arndb.de>
Reviewed-by: Frank Rowand <frank.rowand@sony.com>
Tested-by: Frank Rowand <frank.rowand@sony.com>
Signed-off-by: Rob Herring <robh@kernel.org>