include: axp_pmic: Add missing header guard definition

This header attempted to avoid multiple inclusion using a header guard.
But the preprocessor symbol was never defined, so the guard had no
effect. Fix this by defining the symbol.

Signed-off-by: Samuel Holland <samuel@sholland.org>
Reviewed-by: Andre Przywara <andre.przywara@arm.com>
Signed-off-by: Andre Przywara <andre.przywara@arm.com>