Add a bpf_tethering_headers lib used by the programs and JNI.

This provides a place to store common definitions that can be
used by both the programs and by JNI code in the tethering
module.

Over time much of the tethering-specific code in bpf_shared.h
could move here.

Test: m
Bug: 177884581
Change-Id: I4bf2c762d61cd5db003d59880e6bf7d107ff1a07
diff --git a/Tethering/Android.bp b/Tethering/Android.bp
index 7427481..73175580 100644
--- a/Tethering/Android.bp
+++ b/Tethering/Android.bp
@@ -62,7 +62,10 @@
         "com.android.tethering",
     ],
     min_sdk_version: "30",
-    header_libs: ["bpf_syscall_wrappers"],
+    header_libs: [
+        "bpf_syscall_wrappers",
+        "bpf_tethering_headers",
+    ],
     srcs: [
         "jni/*.cpp",
     ],