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/linker_reloc_iterators.h b/linker/linker_reloc_iterators.h
index 6340924..b162684 100644
--- a/linker/linker_reloc_iterators.h
+++ b/linker/linker_reloc_iterators.h
@@ -26,8 +26,7 @@
  * SUCH DAMAGE.
  */
 
-#ifndef __LINKER_RELOC_ITERATORS_H
-#define __LINKER_RELOC_ITERATORS_H
+#pragma once
 
 #include "linker.h"
 
@@ -173,5 +172,3 @@
   size_t relocation_group_index_;
   rel_t reloc_;
 };
-
-#endif  // __LINKER_RELOC_ITERATORS_H