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: 81d0848fc8d2 ("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>

drivers/of/of_private.h

@@ -55,7 +55,7 @@ static inline int of_property_notify(int action, struct device_node *np,
 }
 #endif /* CONFIG_OF_DYNAMIC */
 
-#ifdef CONFIG_OF_UNITTEST
+#if defined(CONFIG_OF_UNITTEST) && defined(CONFIG_OF_OVERLAY)
 extern void __init unittest_unflatten_overlay_base(void);
 #else
 static inline void unittest_unflatten_overlay_base(void) {};