bgp: add a constant for enhanced route refresh
bgp: add a constant for enhanced route refresh

Signed-off-by: YAMAMOTO Takashi <458bba2e4a8704776b33478100c98529b639cd9e@valinux.co.jp>
Signed-off-by: FUJITA Tomonori <93dac1fe9c4b2a3957982200319981492ad4976e@lab.ntt.co.jp>

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