Fix compiler warnings

This prevents compiler warnings when using libpng 1.6 and GCC 4.8

shared/image-loader.c

@@ -25,6 +25,7 @@
 
 #include <stdlib.h>
 #include <stdio.h>
+#include <string.h>
 #include <jpeglib.h>
 #include <png.h>
 #include <pixman.h>