radvd: add a patch to fix build failure on architectures without sysctl()

Modern architectures such as ARC do not provide the sysctl() system
call, since it is deprecated. However, uClibc still installs
<sys/sysctl.h> in such cases, which defeats radvd check for the
availability of sysctl(). This commit adds a patch to radvd which
improves the sysctl() checking.

Fixes:

  http://autobuild.buildroot.org/results/458/4581c4220adeaebbf6761e3b923088d8de8522d5/



Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>
Reviewed-by: "Yann E. MORIN" <yann.morin.1998@free.fr>