-Lexport-header includes package root.

Looks like this:
// Source: android.hardware.nfc@1.0
// Root: android.hardware:hardware/interfaces

Bug: 33541458
Test: -Lexport-header

Change-Id: I11fe22066aa5f5a0cbee4bb0c46428c3629e7d40
diff --git a/main.cpp b/main.cpp
index 647ba1c..fc6b013 100644
--- a/main.cpp
+++ b/main.cpp
@@ -1001,7 +1001,8 @@
     Formatter out(file);
 
     out << "// This file is autogenerated by hidl-gen. Do not edit manually.\n"
-        << "// Source: " << packageFQName.string() << "\n\n";
+        << "// Source: " << packageFQName.string() << "\n"
+        << "// Root: " << coordinator->getPackageRootOption(packageFQName) << "\n\n";
 
     std::string guard;
     if (forJava) {