Silicon/AMD/StyxSpiFvDxe: declare missing GUID dependency

StyxSpiFvDxe depends on gEfiEventVirtualAddressChangeGuid, but got
away with not declaring it in its INF because of a transitive
dependency. However, this dependency got dropped in core EDK2,
resulting in build failures of the Styx platform.

Signed-off-by: Ard Biesheuvel <ard.biesheuvel@linaro.org>
Reviewed-by: Leif Lindholm <leif.lindholm@linaro.org>