tools/gn/example: Fix the usage of HELLO_SHARED_IMPLEMENTATION.

tools/gn/example/hello_shared.h

Index: tools/gn/example/hello_shared.h
diff --git a/tools/gn/example/hello_shared.h b/tools/gn/example/hello_shared.h
index f62b5eec854180718464438d335c56e45ada2001..7af804b3be4b08a7d2c29d41f9c59d76ddb91016 100644
--- a/tools/gn/example/hello_shared.h
+++ b/tools/gn/example/hello_shared.h
@@ -2,8 +2,8 @@
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
-#ifndef HELLO_SHARED_H_
-#define HELLO_SHARED_H_
+#ifndef TOOLS_GN_EXAMPLE_HELLO_SHARED_H_
+#define TOOLS_GN_EXAMPLE_HELLO_SHARED_H_
 
 #if defined(WIN32)
 
@@ -17,16 +17,16 @@
 
 #else
 
-#if defined(HELLO_IMPLEMENTATION)
+#if defined(HELLO_SHARED_IMPLEMENTATION)
 #define HELLO_EXPORT __attribute__((visibility("default")))
 #define HELLO_EXPORT_PRIVATE __attribute__((visibility("default")))
 #else
 #define HELLO_EXPORT
 #define HELLO_EXPORT_PRIVATE
-#endif  // defined(HELLO_IMPLEMENTATION)
+#endif  // defined(HELLO_SHARED_IMPLEMENTATION)
 
 #endif
 
 HELLO_EXPORT const char* GetSharedText();
 
-#endif  // HELLO_SHARED_H_
+#endif  // TOOLS_GN_EXAMPLE_HELLO_SHARED_H_