Export kernel headers for musl

Musl will reuse bionic's kernel headers.

Bug: 190084016
Test: m libc_musl
Change-Id: Ie750556f9a055984eb9bab41fa61faa965f037e5
diff --git a/libc/Android.bp b/libc/Android.bp
index 764173d..fbf7ec2 100644
--- a/libc/Android.bp
+++ b/libc/Android.bp
@@ -1859,7 +1859,9 @@
 
 cc_library_headers {
     name: "libc_llndk_headers",
-    visibility: ["//visibility:private"],
+    visibility: [
+        "//external/musl",
+    ],
     llndk: {
         llndk_headers: true,
     },