adaptivedemux2: Fix meson .wrap file to name correct dependency

The Meson build fails to build adaptivedemux2 if the user does not have libxml2 installed on the system and visible to pkg-config or cmake. This is because it cannot find the proper fallback. While libxml2_dep as used in the libxml2.wrap file does match the naming conventions used in other GStreamer .wrap files, it does not match the dependency name used in the meson.build file within the libxml2-2.9.7 subproject. This merge request resolves the mismatch and allows adaptivedemux2 to be built properly.