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>
(cherry picked from commit 0737f48c5f8f502cd16123c5251d4ccaaee900ee)
Signed-off-by: Peter Korsgaard <peter@korsgaard.com>
Baruch Siach