perl-gdgraph: fix configure

see http://autobuild.buildroot.org/results/3e1/3e1698b0523dfa46f5b3ee03a22af820f7342c39/build-end.log

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