contrib: bump opendht to 1.6.0rc2

Include the fix for the 2038 bug.

Change-Id: I1a5f37ea4fe15bc0bd4c3949f913c3498b74c1f6

contrib/src/opendht/SHA512SUMS

-7db850afe9eebd580c7be97f591d8d3f36828387a9aedbd3c5ca48212251f9e650131dd6d81cd487ae3e58212f106abf6ea2a16e710297068778ffd25c8ebbd9  opendht-1.6.0rc1.tar.gz
+c12763350d63a63122fcfbe52d3d17644279f24a8ae3154db0e4924f050c4c32c027a238ea79940e647ab538f8aaa87f2bed139f6404551c27ca637e25ebdd86  opendht-1.6.0rc2.tar.gz

contrib/src/opendht/rules.mak

 # OPENDHT
-OPENDHT_VERSION := 1.6.0rc1
+OPENDHT_VERSION := 1.6.0rc2
 OPENDHT_URL := https://github.com/savoirfairelinux/opendht/archive/$(OPENDHT_VERSION).tar.gz
 
 PKGS += opendht