of: unittest: move misplaced function declaration

The overlay_data_apply() declaration is outside of the #ifdef that contains
both the user and the definition, causing a compile-time warning in
some configurations:

drivers/of/unittest.c:48:19: error: 'overlay_data_apply' declared 'static' but never defined [-Werror=unused-function]
 static int __init overlay_data_apply(const char *overlay_name, int *overlay_id);

This moves the declaration into the #ifdef section.

Fixes: 39a751a4 ("of: change overlay apply input data from unflattened to FDT")
Signed-off-by: Arnd Bergmann <arnd@arndb.de>
Signed-off-by: Rob Herring <robh@kernel.org>