http://bugs.gentoo.org/499480

Patch is from ArchLinux git libmng-2.0.2-3

--- libmng_types.h
+++ libmng_types.h
@@ -204,6 +204,7 @@
 #define HAVE_BOOLEAN
 typedef int boolean;
 #endif
+#include <stdio.h>
 #include <jpeglib.h>
 #endif /* MNG_INCLUDE_IJG6B */