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

android/include/linux/udp.h

diff --git a/android/include/linux/udp.h b/android/include/linux/udp.h
new file mode 100644
index 0000000..cfc749f
--- /dev/null
+++ b/android/include/linux/udp.h
@@ -0,0 +1,8 @@
+#pragma once
+
+// Bionic(bionic/libc/kernel/tools/defaults.py) replaces udphdr
+// with __kernel_udphdr and because of that including linux/udp.h
+// is insufficient. Undo the renaming performed by the script.
+#define __kernel_udphdr udphdr
+
+#include_next <linux/udp.h>