apps: propagate fuzzing feature to quiche

This way we can build the apps with quiche in fuzzing mode.
diff --git a/tools/apps/Cargo.toml b/tools/apps/Cargo.toml
index 58db78e..05b8839 100644
--- a/tools/apps/Cargo.toml
+++ b/tools/apps/Cargo.toml
@@ -5,6 +5,10 @@
 edition = "2018"
 publish = false
 
+[features]
+# Enable quiche's fuzzing mode.
+fuzzing = ["quiche/fuzzing"]
+
 [dependencies]
 docopt = "1"
 env_logger = "0.6"