ci/build-linux-old: add missing Werror

Note that deprecated warnings are not disabled on this build, as the
idea is that on "old" one we shouldn't have any depracated warnings
anymore.

Author: kasper93

Diff for: ci/build-linux-old.sh

@@ -9,6 +9,7 @@ git clone https://code.videolan.org/videolan/libplacebo.git \
     --depth=1 --branch v6.338 subprojects/libplacebo \
 
 meson setup build \
+    --werror      \
     -Dlibplacebo:vulkan=disabled \
     -Dlibmpv=true \
     -Dlua=enabled \