buildroot

package/upx: needs zlib

Fixes build issues like, observed on a stripped-down build system:
    compress.cpp:32:18: fatal error: zlib.h: No such file or directory
     #include <zlib.h>
                  ^
    compilation terminated.

Signed-off-by: "Yann E. MORIN" <yann.morin.1998@free.fr>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>