dtc: needs host-bison and host-flex

Fixes:
http://autobuild.buildroot.net/results/000/000a042f6b485d90717d14b93efcf011841c33c3/



Signed-off-by: Gustavo Zacarias <gustavo@zacarias.com.ar>
Acked-by: "Yann E. MORIN" <yann.morin.1998@free.fr>
Signed-off-by: Peter Korsgaard <jacmet@sunsite.dk>