Mark libvold_headers as recovery_available.

recovery modules use <cryptfs.h> (in particular CRYPT_FOOTER_OFFSET).

Test: mmma bootable/recovery
Change-Id: I97b038476ccbeb0a5ac4a6cde86063325bdbb152
diff --git a/Android.bp b/Android.bp
index 55def04..b621f35 100644
--- a/Android.bp
+++ b/Android.bp
@@ -86,6 +86,7 @@
 
 cc_library_headers {
     name: "libvold_headers",
+    recovery_available: true,
     export_include_dirs: ["."],
 }