chrony: fix build with musl

chrony: fix build with musl

The SCM_TIMESTAMPING_PKTINFO macro is missing from the socket.h header
that musl libc provides as of version 1.1.16 that we currently use in
Buildroot. Patch the code to add this macro locally until we update
musl.

Fixes:
http://autobuild.buildroot.net/results/aab/aab3210d6d0c60fb07415d9bbdb09f2d4091446c/
http://autobuild.buildroot.net/results/fc8/fc8440db025f51c176240ca127bc68a367add21c/

Cc: Nathan Lynch <ntl@pobox.com>
Signed-off-by: Baruch Siach <baruch@tkos.co.il>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>