rpm: do not export MAGIC in SDK

This MAGIC variable is used by libmagic (file), and should
be exported by the file recipe.

As the rpm recipe has 'file' in its DEPENDS, its sub-packages
which links to libmagic will be automatically added the 'file'
runtime dependency. More specifically, it's the rpm-build package.
So in case components in nativesdk-rpm-build package uses libmagic,
the nativesdk-file package will be installed and the MAGIC var
will be exported.

Signed-off-by: Chen Qi <Qi.Chen@windriver.com>
Signed-off-by: Luca Ceresoli <luca.ceresoli@bootlin.com>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>