kernel: Add clean headers script to workaround conflicts with bionic

After the "Avoid multiple definitions of sigaction." change in bionic,
we ought to modify our kernel headers to make sure that sigaction struct
is not present in uapi headers.

Change-Id: I15645480e013e79cbcafaac99253368b646b6b11

tools/clean_headers.sh

@@ -0,0 +1,7 @@
+#!/bin/bash
+set -eu
+
+export ANDROID_BUILD_TOP="$PWD"
+
+./bionic/libc/kernel/tools/clean_header.py -u \
+    "$1/usr/include/asm-generic/signal.h"