Fix public import header generation.

PiperOrigin-RevId: 472841214
diff --git a/upbc/protoc-gen-upb.cc b/upbc/protoc-gen-upb.cc
index d04b1f6..24ac89d 100644
--- a/upbc/protoc-gen-upb.cc
+++ b/upbc/protoc-gen-upb.cc
@@ -896,7 +896,7 @@
     if (i == 0) {
       output("/* Public Imports. */\n");
     }
-    output("#include \"$0\"\n", HeaderFilename(file));
+    output("#include \"$0\"\n", HeaderFilename(file->public_dependency(i)));
     if (i == file->public_dependency_count() - 1) {
       output("\n");
     }