stubs missing include for string.h

stubs.cpp gets string.h inherited from private/android_filesystem_config.h
it should not rely on this in the future. The intent is to move fs_config
function into libcutils and thus deprecate any need for string.h in this
include file.

Change-Id: I946ec1979ef5bbb34fbcb4a99bf2cd79280bb2a3
diff --git a/libc/bionic/stubs.cpp b/libc/bionic/stubs.cpp
index f9a31b9..7970e7c 100644
--- a/libc/bionic/stubs.cpp
+++ b/libc/bionic/stubs.cpp
@@ -35,6 +35,7 @@
 #include <pwd.h>
 #include <stdio.h>
 #include <stdlib.h>
+#include <string.h>
 #include <unistd.h>
 
 #include "private/android_filesystem_config.h"