testing/youtube-tui: add missing dep on libsixel-dev

riscv64-alpine-linux-musl/bin/ld: cannot find -lsixel: No such file or directory

testing/youtube-tui/APKBUILD

@@ -6,7 +6,7 @@ pkgdesc="Aesthetically pleasing YouTube TUI written in Rust"
 url="https://siriusmart.github.io/youtube-tui/"
 arch="all"
 license="GPL-3.0-or-later"
-makedepends="cargo cargo-auditable mpv-dev"
+makedepends="cargo cargo-auditable mpv-dev libsixel-dev"
 source="https://github.com/Siriusmart/youtube-tui/archive/v$pkgver/youtube-tui-$pkgver.tar.gz"
 
 prepare() {