Fix build of pdfium_diff

Commit af18cb3 didn't build standalone.
TBR=thestig@chromium.org

Review URL: https://codereview.chromium.org/901313002
diff --git a/samples/image_diff_png.h b/samples/image_diff_png.h
index ea77e9a..9febb37 100644
--- a/samples/image_diff_png.h
+++ b/samples/image_diff_png.h
@@ -5,6 +5,8 @@
 #ifndef TOOLS_IMAGEDIFF_IMAGE_DIFF_PNG_H_
 #define TOOLS_IMAGEDIFF_IMAGE_DIFF_PNG_H_
 
+#include <stdlib.h>  // for size_t.
+
 #include <vector>
 
 namespace image_diff_png {