idmap2: fix cpplint warnings

Bug: 189963636
Test: frameworks/base/cmds/idmap2/static-checks.sh
Change-Id: I6da18cdbbd2fc33db711696445230a9f316dc143
diff --git a/cmds/idmap2/libidmap2/ResourceUtils.cpp b/cmds/idmap2/libidmap2/ResourceUtils.cpp
index e809bf1..32c04d2 100644
--- a/cmds/idmap2/libidmap2/ResourceUtils.cpp
+++ b/cmds/idmap2/libidmap2/ResourceUtils.cpp
@@ -17,6 +17,7 @@
 #include "idmap2/ResourceUtils.h"
 
 #include <memory>
+#include <string>
 
 #include "androidfw/StringPiece.h"
 #include "androidfw/Util.h"