Update libdns_la_LIBADD rather than libdns_la_LDFLAGS

the wrong macro was being update with MAXMINDDB_LIBS making
it difficult to adjust link order.

(cherry picked from commit 5b516101)
65 jobs for !6557 with 3445-update-libdns_la_libadd-instead-of-libdns_la_ldflags-v9_18 in 19 minutes and 43 seconds (queued for 7 seconds)
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