buildroot

perl-gd: bump to version 2.56

perl-gd now supplies a Build.PL, but this one is not cross-compilation
friendly. So, we still use Makefile.PL like in previous version by using
the new <pkg>_PREFER_INSTALLER variable.

Signed-off-by: Francois Perrad <francois.perrad@gadz.org>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>