ipn/conffile: don't depend on hujson on iOS/Android

Fixes #13772

Change-Id: I3ae03a5ee48c801f2e5ea12d1e54681df25d4604
Signed-off-by: Brad Fitzpatrick <bradfitz@tailscale.com>

tstest/iosdeps/iosdeps_test.go

@@ -23,6 +23,7 @@ func TestDeps(t *testing.T) {
 			"database/sql/driver":                 "iOS doesn't use an SQL database",
 			"github.com/google/uuid":              "see tailscale/tailscale#13760",
 			"tailscale.com/clientupdate/distsign": "downloads via AppStore, not distsign",
+			"github.com/tailscale/hujson":         "no config file support on iOS",
 		},
 	}.Check(t)
 }