libbsd: correct license file reference

Fixes autobuilder error:
http://autobuild.buildroot.net/results/ccd/ccd2f3fe69a3e0321685daf95f85eca5ba2d64cf/



[Peter: use COPYING instead]
Signed-off-by: Gustavo Zacarias <gustavo@zacarias.com.ar>
Signed-off-by: Peter Korsgaard <jacmet@sunsite.dk>