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_soinfo.h b/linker/linker_soinfo.h
index 447c7c3..6e1f005 100644
--- a/linker/linker_soinfo.h
+++ b/linker/linker_soinfo.h
@@ -26,8 +26,7 @@
  * SUCH DAMAGE.
  */
 
-#ifndef __LINKER_SOINFO_H
-#define __LINKER_SOINFO_H
+#pragma once
 
 #include <link.h>
 
@@ -386,5 +385,3 @@
     }
   }
 }
-
-#endif  /* __LINKER_SOINFO_H */