Fix build in 5.1 tree. to_string doesn't exist in bionic in 5.1.

Change-Id: If10b76f7a2b695c4be470812e048c66f5920d798
diff --git a/twrp.cpp b/twrp.cpp
index cf519bb..37053d2 100755
--- a/twrp.cpp
+++ b/twrp.cpp
@@ -45,7 +45,7 @@
 #include "partitions.hpp"
 
 #ifdef USE_OLD_BASE_INCLUDE
-#include <base/strings.h>
+#include <strings.h>
 #else
 #include <android-base/strings.h>
 #endif