ArmVirtPkg/ArmVirt.dsc.inc: Add VariableFlashInfoLib

ArmVirtPkg/ArmVirt.dsc.inc: Add VariableFlashInfoLib

REF:https://bugzilla.tianocore.org/show_bug.cgi?id=3479

Adds an instance of VariableFlashInfoLib to the platform build as
it is a new library class introduced in MdeModulePkg.

Cc: Ard Biesheuvel <ardb+tianocore@kernel.org>
Cc: Leif Lindholm <quic_llindhol@quicinc.com>
Cc: Sami Mujawar <sami.mujawar@arm.com>
Cc: Gerd Hoffmann <kraxel@redhat.com>
Cc: Julien Grall <julien@xen.org>
Signed-off-by: Michael Kubacki <michael.kubacki@microsoft.com>
Acked-by: Ard Biesheuvel <ardb@kernel.org>
Reviewed-by: Liming Gao <gaoliming@byosoft.com.cn>
Reviewed-by: Sami Mujawar <sami.mujawar@arm.com>