Switch the rest of our internal headers to #pragma once.

We've been using #pragma once for new internal files, but let's be more bold.

Bug: N/A
Test: builds
Change-Id: I7e2ee2730043bd884f9571cdbd8b524043030c07
diff --git a/linker/linked_list.h b/linker/linked_list.h
index ed2b150..048ea4d 100644
--- a/linker/linked_list.h
+++ b/linker/linked_list.h
@@ -26,8 +26,7 @@
  * SUCH DAMAGE.
  */
 
-#ifndef __LINKED_LIST_H
-#define __LINKED_LIST_H
+#pragma once
 
 #include "private/bionic_macros.h"
 
@@ -259,5 +258,3 @@
   LinkedListEntry<T>* tail_;
   DISALLOW_COPY_AND_ASSIGN(LinkedList);
 };
-
-#endif // __LINKED_LIST_H