of/unittest: Remove unnecessary module.h header inclusion

The OF_UNITTEST Kconfig symbol is bool so this unittest can only be
built-in and not build as a module. Also, nothing defined in this
header file used so is not necessary to include it.

Signed-off-by: Javier Martinez Canillas <javier@osg.samsung.com>
Signed-off-by: Rob Herring <robh@kernel.org>