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_dlwarning.h b/linker/linker_dlwarning.h
index 68827f7..af26824 100644
--- a/linker/linker_dlwarning.h
+++ b/linker/linker_dlwarning.h
@@ -26,8 +26,7 @@
  * SUCH DAMAGE.
  */
 
-#ifndef __LINKER_DLWARNING_H
-#define __LINKER_DLWARNING_H
+#pragma once
 
 void add_dlwarning(const char* sopath, const char* message, const char* value = nullptr);
 
@@ -36,5 +35,3 @@
 // is used to avoid forcing user into saving the message
 // to a global variable.
 void get_dlwarning(void* user_data, void (*f)(void*, const char*));
-
-#endif  /* __LINKER_DLWARNING_H */