build API change: update usage of addCSourceFiles

pkg/libxml2/build.zig

@@ -95,7 +95,7 @@ pub fn build(b: *std.Build) !void {
     }
 
     lib.addCSourceFiles(.{
-        .dependency = upstream,
+        .root = upstream.path(""),
         .files = srcs,
         .flags = flags.items,
     });