Add rule for AVB signing an image

This is used multiple times by Microdroid to generate signed images.

Test: build and microdroid tests
Bug: 234564414
Bug: 185115783
Change-Id: I7d0b2c9907913c8a42a02f9951f341d08b585b8e
diff --git a/filesystem/Android.bp b/filesystem/Android.bp
index 857dfa7..dfcd405 100644
--- a/filesystem/Android.bp
+++ b/filesystem/Android.bp
@@ -12,6 +12,7 @@
         "soong-linkerconfig",
     ],
     srcs: [
+        "avb_add_hash_footer.go",
         "bootimg.go",
         "filesystem.go",
         "logical_partition.go",