otafault: Add the missing #include <string>.

Test: mmma bootable/recovery
Change-Id: If7610ba55a2d2dd4257ff38c44d252c9c6bbfe40
diff --git a/otafault/ota_io.cpp b/otafault/ota_io.cpp
index 1308973..63ef18e 100644
--- a/otafault/ota_io.cpp
+++ b/otafault/ota_io.cpp
@@ -26,6 +26,7 @@
 
 #include <map>
 #include <mutex>
+#include <string>
 
 #include <android-base/thread_annotations.h>