Add linux/udp.h

Bionic(bionic/libc/kernel/tools/defaults.py) replaces udphdr
with __kernel_udphdr and because
of that build targets like hardware/google/apf can't compile
with musl just by including linux/udp.h. This patch undoes the renaming.

See aosp/3467528 for more context.

Change-Id: Id9a640664769cf82537105f3608d8bb0f1b2f8ef