[C++] Fix newlines in $(info/warning/error)

Change-Id: Ia20a1ef563a6871ed843b9388fe27e87b8bd7020
diff --git a/strutil.h b/strutil.h
index 8c47f1c..caeb8de 100644
--- a/strutil.h
+++ b/strutil.h
@@ -138,4 +138,6 @@
 
 string ConcatDir(StringPiece b, StringPiece n);
 
+string EchoEscape(const string str);
+
 #endif  // STRUTIL_H_