vpci: fix function attributes for vpci_process_pending

vpci: fix function attributes for vpci_process_pending

vpci_process_pending is defined with different attributes, e.g.
with __must_check if CONFIG_HAS_VPCI enabled and not otherwise.
Fix this by defining both of the definitions with __must_check.

Fixes: 14583a590783 ("7fbb096bf345 kconfig: don't select VPCI if building a shim-only binary")
Signed-off-by: Oleksandr Andrushchenko <oleksandr_andrushchenko@epam.com>
Reviewed-by: Julien Grall <jgrall@amazon.com>
Reviewed-by: Roger Pau Monné <roger.pau@citrix.com>