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/malloc_debug.h b/libc/malloc_debug/malloc_debug.h
index 4a1e8da..18a6c2c 100644
--- a/libc/malloc_debug/malloc_debug.h
+++ b/libc/malloc_debug/malloc_debug.h
@@ -26,8 +26,7 @@
  * SUCH DAMAGE.
  */
 
-#ifndef MALLOC_DEBUG_H
-#define MALLOC_DEBUG_H
+#pragma once
 
 #include <stdint.h>
 
@@ -70,5 +69,3 @@
 constexpr size_t FREE_TRACK_MEM_BUFFER_SIZE = 4096;
 
 extern const MallocDispatch* g_dispatch;
-
-#endif // MALLOC_DEBUG_H