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/bionic/grp_pwd_file.h b/libc/bionic/grp_pwd_file.h
index ae8cd98..93dd852 100644
--- a/libc/bionic/grp_pwd_file.h
+++ b/libc/bionic/grp_pwd_file.h
@@ -26,8 +26,7 @@
  * SUCH DAMAGE.
  */
 
-#ifndef GRP_PWD_FILE_H
-#define GRP_PWD_FILE_H
+#pragma once
 
 #include <grp.h>
 #include <pwd.h>
@@ -93,5 +92,3 @@
  private:
   MmapFile mmap_file_;
 };
-
-#endif