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/debug_disable.h b/libc/malloc_debug/debug_disable.h
index 9edb4df..0049595 100644
--- a/libc/malloc_debug/debug_disable.h
+++ b/libc/malloc_debug/debug_disable.h
@@ -26,8 +26,7 @@
  * SUCH DAMAGE.
  */
 
-#ifndef MALLOC_DEBUG_DISABLE_H
-#define MALLOC_DEBUG_DISABLE_H
+#pragma once
 
 #include <sys/cdefs.h>
 
@@ -60,5 +59,3 @@
 
   DISALLOW_COPY_AND_ASSIGN(ScopedDisableDebugCalls);
 };
-
-#endif  // MALLOC_DEBUG_DISABLE_H