Contribs: update libass to 0.12.0

(cherry picked from commit 5e9a545dd63816b3fa8d45665288b8fb82d68e75)
Signed-off-by: Jean-Baptiste Kempf <jb@videolan.org>

contrib/src/ass/SHA512SUMS

-84ab20c3f9ede1c68ae91eb23c7047657abf98d3c0567136e39d165e04075bb72a6a95036cfe42274e87ef91b3cf07fbb8e81ed6f052b4248a9763b296d63abb  libass-0.11.2.tar.gz
+47d6b9b43c549c7f5472c70b32095686d3f0f6c8a16b4c08e4b342532ce51f583855ab43fea93d2dbe8919f85f0ef1acad07d9f43f34e12550f9a4030132c45f  libass-0.12.0.tar.gz

contrib/src/ass/rules.mak

 # ASS
-ASS_VERSION := 0.11.2
+ASS_VERSION := 0.12.0
 ASS_URL := https://github.com/libass/libass/releases/download/$(ASS_VERSION)/libass-$(ASS_VERSION).tar.gz
 
 PKGS += ass