package/poco: disable build for riscv

package/poco: disable build for riscv

poco does not support the riscv target.

Fixes:
http://autobuild.buildroot.net/results/9a8/9a8213c502df53222eafc3ecd2fcfa36db20950b/
http://autobuild.buildroot.net/results/dd4/dd48cac70e8cb697b42ee51561902df81edcea40/
http://autobuild.buildroot.net/results/030/030c6cc8e2a59b015f8f3793d76234a2ef4ab772/

Cc: Yegor Yefremov <yegorslists@googlemail.com>
Signed-off-by: Baruch Siach <baruch@tkos.co.il>
Signed-off-by: Arnout Vandecappelle (Essensium/Mind) <arnout@mind.be>