contrib: added boost header-only dependency for yaml-cpp

Will only be needed if building contrib on a system that doesn't package yaml-cpp.

Change-Id: I3ba71b52349ff33adfa324d7830a18bc953a68a6

daemon/contrib/src/boost-headers/SHA512SUMS

+a0ac451a51c5b183ee5625da66ccdbbc5a079d71c3d1bd7570e1e5f30a3f2f852a8e8237a60a3419ee72bcffdf4e1e7867d9d6ab91cf314db95df2142b70c008  boost-headers.tar.gz

daemon/contrib/src/boost-headers/rules.mak

+# Boost headers (needed for building yaml-cpp)
+BOOST_HEADERS_URL = https://gitlab.savoirfairelinux.com/sfl-ports/boost-headers/repository/archive.tar.gz
+
+$(TARBALLS)/boost-headers.tar.gz:
+	$(call download,$(BOOST_HEADERS_URL))
+
+.sum-boost-headers: boost-headers.tar.gz
+
+boost-headers: boost-headers.tar.gz .sum-boost-headers
+	$(UNPACK)
+	mv boost-headers.git boost
+
+.boost-headers: boost-headers
+	touch $@