Export android_filesystem_config.h as a filegroup

bionic/libc/Android.bp needs to reference android_filesystem_config.h,
export it using a filegroup module.

Bug: 34283327
Test: builds
Change-Id: Id7650f3b64d7eb52f945f538d05b691b8b3c3fcb
diff --git a/Android.bp b/Android.bp
index b44c296..c6f6251 100644
--- a/Android.bp
+++ b/Android.bp
@@ -1 +1,6 @@
+filegroup {
+    name: "android_filesystem_config_header",
+    srcs: ["include/private/android_filesystem_config.h"],
+}
+
 subdirs = ["*"]