ethtool: pse-pd: Add missing linux/export.h include

Fix missing linux/export.h header include in net/ethtool/pse-pd.c to resolve
build warning reported by the kernel test robot.

Reported-by: kernel test robot <lkp@intel.com>
Closes: https://lore.kernel.org/oe-kbuild-all/202506200024.T3O0FWeR-lkp@intel.com/


Signed-off-by: Kory Maincent <kory.maincent@bootlin.com>
Link: https://patch.msgid.link/20250619162547.1989468-1-kory.maincent@bootlin.com


Signed-off-by: Jakub Kicinski <kuba@kernel.org>