Create include/linux/export.h with stub definitions for EXPORT_SYMBOL
and EXPORT_SYMBOL_GPL macros. This matches the Linux kernel's header
organization where export.h is a separate file.
Update compat.h to include export.h and remove the duplicate
definitions.
Co-developed-by: Claude Opus 4.5 <noreply@anthropic.com>
Signed-off-by: Simon Glass <simon.glass@canonical.com>
