conf: fix subtle bug in rust2rpm.conf validation

The "default" feature is always implicitly defined, even if not
explicitly listed in the crate metadata, so it's always valid.

rust2rpm/conf.py

@@ -97,6 +97,8 @@ class Rust2RpmConf:
         ]
         for feature in features:
             valid_keys.append(f"lib+{feature}.requires")
+        if "default" not in features:
+            valid_keys.append(f"lib+default.requires")
 
         # check section names
         for section in conf.keys():