[C++] Improve NormalizePath

So now NormalizePath("../foo") will be "../foo" and "/../foo"
will be "/foo".
diff --git a/strutil.h b/strutil.h
index 2dd229c..8c47f1c 100644
--- a/strutil.h
+++ b/strutil.h
@@ -118,7 +118,7 @@
 StringPiece Basename(StringPiece s);
 StringPiece GetExt(StringPiece s);
 StringPiece StripExt(StringPiece s);
-void NormalizePath(string* o, size_t start_index = 0);
+void NormalizePath(string* o);
 void AbsPath(StringPiece s, string* o);
 
 size_t FindOutsideParen(StringPiece s, char c);