Small cleaning.

src/image/png/PngReader.cpp

@@ -8,6 +8,7 @@
 
 #include "ZlibEncoder.h"
 #include "CyclicRedundancyChecker.h"
+#include "StringUtils.h"
 
 #include <iostream>
 #include <sstream>

src/image/png/PngWriter.cpp

@@ -5,6 +5,7 @@
 #include "BufferBitStream.h"
 #include "OutputBitStream.h"
 #include "ImageBitStream.h"
+#include "StringUtils.h"
 
 #include "PngFilter.h"
 #include "Lz77Encoder.h"