Fix compiler warnings

This prevents compiler warnings when using libpng 1.6 and GCC 4.8
diff --git a/shared/image-loader.c b/shared/image-loader.c
index c9f15d4..9f65dea 100644
--- a/shared/image-loader.c
+++ b/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>