gd: fix pthread related static linking issue for utilities

gd forgets to link utilities with -pthread even though it uses pthreads,
causing linking errors with static linking.

Fixes http://autobuild.buildroot.net/results/156/1564b8de7785c1a756bead1a4160a2b6e2a2243e/



Signed-off-by: Peter Korsgaard <peter@korsgaard.com>