meson: Correct capability check for SunRPC on *BSD #279

Job	Run time
Build Netatalk container	1m 0s
Build Netatalk testsuite container	1m 12s
Netatalk container dry run	3s
AFP login test - AFP 3.4	8s
AFP spec test (Readonly) - AFP 3.4	7s
AFP spec test (Readonly) - AFP 2.1	7s
AFP login test - AFP 2.1	7s
AFP spec test - AFP 2.1	58s
AFP spec test - AFP 2.2	57s
AFP spec test - AFP 3.0	1m 7s
AFP spec test - AFP 3.1	1m 6s
AFP spec test - AFP 3.2	1m 6s
AFP spec test - AFP 3.4	1m 7s
AFP spec test - AFP 3.3	1m 7s
	10m 12s

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