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/malloc_info.h b/libc/bionic/malloc_info.h
index 5fffae9..257fec1 100644
--- a/libc/bionic/malloc_info.h
+++ b/libc/bionic/malloc_info.h
@@ -14,8 +14,7 @@
  * limitations under the License.
  */
 
-#ifndef LIBC_BIONIC_MALLOC_INFO_H_
-#define LIBC_BIONIC_MALLOC_INFO_H_
+#pragma once
 
 #include <malloc.h>
 #include <sys/cdefs.h>
@@ -28,5 +27,3 @@
 __LIBC_HIDDEN__ struct mallinfo __mallinfo_bin_info(size_t, size_t);
 
 __END_DECLS
-
-#endif // LIBC_BIONIC_MALLOC_INFO_H_