package/libfreeimage: fix C++11 compliance with gcc-6

Like in cbe01f67, 38249d56, 97fe953b, fix C++11 compliance regarding
narrowing types.

Fixes:
    http://autobuild.buildroot.org/results/081/0811531872f69f9febbdc482dfbdd7fb5c35d1c8/
    http://autobuild.buildroot.org/results/90c/90c9cb4ffe705b8c91f4fb602e33e2ba9bcdee77/


    and a few others...

[Peter: use signed char for src[], fix dos/unix new lines]
Signed-off-by: "Yann E. MORIN" <yann.morin.1998@free.fr>
Signed-off-by: Peter Korsgaard <peter@korsgaard.com>