selftests/bpf: fix compile errors with older glibc

Wang YanQing bpf lkml 1 version Apr 17, 2019 In review

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#	Subject and files	Size	Replies	Applied
1/1	selftests/bpf: fix compile errors with older glibc`tools/testing/selftests/bpf/progs/test_tcpbpf_kern.c` `tools/testing/selftests/bpf/progs/test_tcpnotify_kern.c`	+2 -2	4 replies

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