Fix libmng test.

libmng pulls in libjpeg. In turn libjpeg requires that code that uses it should
include stdio.h [0].

[0] <https://bugs.debian.org/cgi-bin/bugreport.cgi?bug=461602#10

>

So adding the corresponding include before libmng solves the issue.

Change-Id: I7cfbda727ed5b71b668d6597358a5e9c7d8918bb
Reviewed-by: Dmitry Shachnev <mitya57@gmail.com>
Reviewed-by: Oswald Buddenhagen <oswald.buddenhagen@digia.com>
Reviewed-by: Lisandro Damián Nicanor Pérez Meyer <perezmeyer@gmail.com>

config.tests/libmng/libmng.cpp

@@ -31,6 +31,7 @@
 **
 ****************************************************************************/
 
+#include <stdio.h>
 #include <libmng.h>
 
 int main(int, char **)