package/perl-gd: needs perl-extutils-pkgconfig

Commit 3a291be2 forgot to add
perl-extutils-pkgconfig dependency

Fixes:
 - http://autobuild.buildroot.org/results/e590f1990180eae21512b23b884755e105a4c588



Signed-off-by: Fabrice Fontaine <fontaine.fabrice@gmail.com>
Acked-by: Francois Perrad <francois.perrad@gadz.org>
Signed-off-by: Arnout Vandecappelle (Essensium/Mind) <arnout@mind.be>