Fix compilation fail (cstring include)

Compilation fail:

[ 8/372] cxx: src/mesh.cpp -> build/src/mesh.cpp.1.o
../src/image-reader.cpp: In member function ‘virtual bool PNGReader::nextRow(unsigned char*)’:
../src/image-reader.cpp:78:71: error: ‘memcpy’ was not declared in this scope
         memcpy(dst, priv_->rows[priv_->current_row], priv_->row_stride);

Fix (apllied in Gentoo)

diff -Naur a/glmark2/trunk/src/image-reader.cpp b/glmark2/trunk/src/image-reader.cpp
--- a/glmark2/trunk/src/image-reader.cpp 2013-08-07 16:21:26.655989291 +0200
+++ b/glmark2/trunk/src/image-reader.cpp 2013-08-07 16:20:29.053983661 +0200
@@ -21,6 +21,7 @@
  */
 #include <png.h>
 #include <jpeglib.h>
+#include <cstring>
 #include <memory>

 #include "image-reader.h"