openntpd: fix musl build with older kernel headers

openntpd: fix musl build with older kernel headers

musl provides its own SYS_getrandom definition, but not GRND_NONBLOCK.
This breaks the build with kernel headers older than v3.17. Add a patch
adding a local definition of GRND_NONBLOCK to fix the build.

The getentropy_linux.c file is in upstream tarball, but not in its git
repository. It originates from OpenBSD. For this reason the patch is
against the tarball, but not git formatted.

Fixes:
http://autobuild.buildroot.net/results/623/6235ddc095bc1984439e7339d590b11581a3e257/
http://autobuild.buildroot.net/results/648/648e7c91edcf893fa780de9bb44c05f5f842a2e8/
http://autobuild.buildroot.net/results/7da/7da71263a6e9b780031f1f6185c760d11dfc994e/

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