contrib: bump opendht -> 0.6.3

* Add Microsoft compiler support;
* Work around the issue compiling on Apple's LLVM 7.3.0;
* Add Network id feature;
* License OpenDHT compatible with GPL>=3;
* Adding cryptographic API;
* Consistent library naming between diffrent build tools.

Change-Id: I4e7a53619d89f7b2034326ea8d5f75848335c533
Tuleap: #556

contrib/src/opendht/rules.mak

 # OPENDHT
-OPENDHT_VERSION := 0.6.2
+OPENDHT_VERSION := 0.6.3
 OPENDHT_URL := https://github.com/savoirfairelinux/opendht/archive/$(OPENDHT_VERSION).tar.gz
 
 PKGS += opendht