llext: disable EXPORT_SYMBOL() when CONFIG_LLEXT=n

EXPORT_SYMBOL() isn't needed when CONFIG_LLEXT is deselected, it only
needlessly creates objects, that make the resulting image larger.
Make it a NOP when CONFIG_LLEXT=n.

Signed-off-by: Guennadi Liakhovetski <guennadi.liakhovetski@linux.intel.com>