main/tmux: add missing secfixes info

See: #12094

main/tmux/APKBUILD

@@ -12,6 +12,9 @@ makedepends="autoconf automake libevent-dev ncurses-dev"
 subpackages="$pkgname-doc"
 source="https://github.com/tmux/tmux/releases/download/$pkgver/tmux-$pkgver.tar.gz"
 
+# secfixes:
+#   3.1c-r0:
+#     - CVE-2020-27347
 
 build() {
 	./configure \