dillo: zlib is a mandatory dependency

Fixes:
http://autobuild.buildroot.net/results/10f/10f7ba386a13753c942139b8bc13ca74d8932f3a/



Also reorder variables a bit and ditch duplicate addition.

Signed-off-by: Gustavo Zacarias <gustavo@zacarias.com.ar>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>