package/nanomsg: fix legal-info failure

After version bump, the sha256 has been updated
while the COPYING file remains unchanged.

Fixes:
 - http://autobuild.buildroot.net/results/ec1cb0216ebedc5c7bd2c8719460db28b666a24f/



Signed-off-by: Pierre-Jean Texier <pjtexier@koncepto.io>
Signed-off-by: Peter Korsgaard <peter@korsgaard.com>