Add se_compat_cil module

Installs backwards compatibility cil files.

Bug: 183362912
Test: Presubmit
Test: Add a $(ver).compat.cil under SYSTEM_EXT_PRIVATE_SEPOLICY_DIR and
  verify the file is installed under /system_ext/etc/selinux/mapping/
Change-Id: I5e2c6b8dfa8df431edfe96f29daae463b130367f
diff --git a/build/soong/Android.bp b/build/soong/Android.bp
index aa6ad71..6a52fe5 100644
--- a/build/soong/Android.bp
+++ b/build/soong/Android.bp
@@ -33,6 +33,7 @@
     srcs: [
         "build_files.go",
         "cil_compat_map.go",
+        "compat_cil.go",
         "filegroup.go",
         "policy.go",
         "selinux.go",