declare the libsodium dependency

Summary: Folly has an optional libsodium dependency.  Declare that in folly getdeps manifest so that the relevant code is built and tested in CI.

Reviewed By: yfeldblum

Differential Revision: D34028478

fbshipit-source-id: 5e7c2f86602b94a3a975907adf8bd04f49ec33c0

build/fbcode_builder/manifests/folly

@@ -17,6 +17,7 @@ glog
 googletest
 boost
 libevent
+libsodium
 double-conversion
 fmt
 lz4