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/backtrace.h b/libc/malloc_debug/backtrace.h
index f570873..92ea237 100644
--- a/libc/malloc_debug/backtrace.h
+++ b/libc/malloc_debug/backtrace.h
@@ -26,8 +26,7 @@
  * SUCH DAMAGE.
  */
 
-#ifndef MALLOC_DEBUG_BACKTRACE_H
-#define MALLOC_DEBUG_BACKTRACE_H
+#pragma once
 
 #include <stdint.h>
 #include <sys/cdefs.h>
@@ -39,5 +38,3 @@
 size_t backtrace_get(uintptr_t* frames, size_t frame_count);
 void backtrace_log(const uintptr_t* frames, size_t frame_count);
 std::string backtrace_string(const uintptr_t* frames, size_t frame_count);
-
-#endif // MALLOC_DEBUG_BACKTRACE_H