Contribs: update harfbuzz to 2.6.8

Last of the 2.6.x series

(cherry picked from commit c4cc7329)
Signed-off-by: Hugo Beauzée-Luyssen <hugo@beauzee.fr>

contrib/src/harfbuzz/SHA512SUMS

-d8664bb64fda11ff7646693070637e3827f8b3d1de50e11ecf108ce4d19c878b26b2ba4cff278da6e6cc0cb431e1630d9eaa7c32a9bebb9655a7aa8dabf7114f  harfbuzz-2.6.4.tar.xz
+651b23d7d4fab6fef472ee57db39bbaebc31c2f40ea3a482c1499ad1a7b549b86a2bccbe9da191c8e9ecdf464191dd3c9a7485546b51695ad8ab3c0329732d9d  harfbuzz-2.6.8.tar.xz

contrib/src/harfbuzz/rules.mak

 # HARFBUZZ
 
-HARFBUZZ_VERSION := 2.6.4
-HARFBUZZ_URL := http://www.freedesktop.org/software/harfbuzz/release/harfbuzz-$(HARFBUZZ_VERSION).tar.xz
+HARFBUZZ_VERSION := 2.6.8
+HARFBUZZ_URL := https://github.com/harfbuzz/harfbuzz/releases/download/$(HARFBUZZ_VERSION)/harfbuzz-$(HARFBUZZ_VERSION).tar.xz
 PKGS += harfbuzz
 ifeq ($(call need_pkg,"harfbuzz"),)
 PKGS_FOUND += harfbuzz