OvmfPkg/PlatformBootManagerLib: guard the definition of ARRAY_SIZE

OvmfPkg/PlatformBootManagerLib: guard the definition of ARRAY_SIZE

In one of the next patches, we'll introduce ARRAY_SIZE in
"MdePkg/Include/Base.h". In order to proceed in small steps, make the
module-local definition of ARRAY_SIZE conditional. This way the
introduction of the macro under MdePkg will silently switch this module
over (after which we can remove the module-local definition completely).

Cc: Jordan Justen <jordan.l.justen@intel.com>
Contributed-under: TianoCore Contribution Agreement 1.0
Signed-off-by: Laszlo Ersek <lersek@redhat.com>
Reviewed-by: Jordan Justen <jordan.l.justen@intel.com>