Meson: Fix libxml2 fallback

The variable xml2lib_dep does not exist. The correct name is already in
the wrap file.

Part-of: <https://gitlab.freedesktop.org/gstreamer/gstreamer/-/merge_requests/3136>

subprojects/gst-plugins-bad/ext/dash/meson.build

@@ -35,7 +35,7 @@ dash_sources = [
 
 xml2_dep = dependency('libxml-2.0',
   version : '>= 2.8',
-  fallback : ['libxml2', 'xml2lib_dep'],
+  fallback : 'libxml2',
   required : get_option('dash')
 )