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_cfi.h b/linker/linker_cfi.h
index e54554f..f3300d6 100644
--- a/linker/linker_cfi.h
+++ b/linker/linker_cfi.h
@@ -26,8 +26,7 @@
  * SUCH DAMAGE.
  */
 
-#ifndef _LINKER_CFI_H_
-#define _LINKER_CFI_H_
+#pragma once
 
 #include "linker.h"
 #include "linker_debug.h"
@@ -103,5 +102,3 @@
 };
 
 CFIShadowWriter* get_cfi_shadow();
-
-#endif // _LINKER_CFI_H_