Clean up <stdlib.h> slightly.

Interestingly, this mostly involves cleaning up our implementation of
various <string.h> functions.

Change-Id: Ifaef49b5cb997134f7bc0cc31bdac844bdb9e089
diff --git a/libc/bionic/scandir.cpp b/libc/bionic/scandir.cpp
index 28b4ed0..ee62fee 100644
--- a/libc/bionic/scandir.cpp
+++ b/libc/bionic/scandir.cpp
@@ -18,6 +18,7 @@
 
 #include <errno.h>
 #include <stdlib.h>
+#include <string.h>
 
 #include "private/bionic_macros.h"
 #include "private/ScopedReaddir.h"