Define linker.config.json as a filegroup

The linker.config.json file is required for soong to generate a
system image.

Bug: 365670526
Test: lunch aosp_cf_x86_64_phone_soong_system-trunk_staging-userdebug
      && m
Change-Id: I86389e4143cd2fc08b812d50962bf7ec007894ab
diff --git a/rootdir/Android.bp b/rootdir/Android.bp
index 5f0a2c3..44acbba 100644
--- a/rootdir/Android.bp
+++ b/rootdir/Android.bp
@@ -50,10 +50,15 @@
     recovery_available: true,
 }
 
+filegroup {
+    name: "system_linker_config_json_file",
+    srcs: ["etc/linker.config.json"],
+}
+
 // TODO(b/147210213) Generate list of libraries during build and fill in at build time
 linker_config {
     name: "system_linker_config",
-    src: "etc/linker.config.json",
+    src: ":system_linker_config_json_file",
     installable: false,
 }