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.

(cherry picked from qtimageformats/08f40bc0f00716c3a8cbda70ed00dc2f16515e85)
Change-Id: Ib249fa9d867536c19be8e376f6c5595080cab006
Reviewed-by: Dmitry Shachnev <mitya57@gmail.com>
Reviewed-by: Lars Knoll <lars.knoll@digia.com>