[dpdk-ci] ABI check vs 21.05

[Thomas Monjalon]

Hi all,

DPDK 21.05 was released and we are now in the 21.08 cycle,
so it's the right time to update the ABI checks
to compare with DPDK 21.05 ABI (numbered 21.2).

The config to update is:
	export DPDK_ABI_REF_VERSION=v21.05