MdeModulePkg/RegularExpressionDxe: 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).