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/libc/malloc_debug/GuardData.h b/libc/malloc_debug/GuardData.h
index ddf6ce1..7b21671 100644
--- a/libc/malloc_debug/GuardData.h
+++ b/libc/malloc_debug/GuardData.h
@@ -26,8 +26,7 @@
  * SUCH DAMAGE.
  */
 
-#ifndef DEBUG_MALLOC_GUARDDATA_H
-#define DEBUG_MALLOC_GUARDDATA_H
+#pragma once
 
 #include <stdint.h>
 #include <string.h>
@@ -93,5 +92,3 @@
 
   DISALLOW_COPY_AND_ASSIGN(RearGuardData);
 };
-
-#endif // DEBUG_MALLOC_GUARDDATA_H