Fix header include guards for fml/thread_local.h (#8721)

While working on #8659, I had planned on renaming this file to
thread_local_unique_ptr.h, but decided against that at the last minute
before sending for review. However, when reverting the file rename, I
forgot to also revert the header guard change.

fml/thread_local.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 FLUTTER_FML_THREAD_LOCAL_UNIQUE_PTR_H_
-#define FLUTTER_FML_THREAD_LOCAL_UNIQUE_PTR_H_
+#ifndef FLUTTER_FML_THREAD_LOCAL_H_
+#define FLUTTER_FML_THREAD_LOCAL_H_
 
 #include <memory>
 
@@ -84,4 +84,4 @@ class ThreadLocalUniquePtr {
 
 }  // namespace fml
 
-#endif  // FLUTTER_FML_THREAD_LOCAL_UNIQUE_PTR_H_
+#endif  // FLUTTER_FML_THREAD_LOCAL_H_