libutil: Sprinkle some noexcept on LRUCache::{size,clear}

The underlying containers are already noexcept to destroy and dtors are
noexcept in general.

src/libutil/include/nix/util/lru-cache.hh

@@ -105,12 +105,12 @@ public:
         return value;
     }
 
-    size_t size() const
+    size_t size() const noexcept
     {
         return data.size();
     }
 
-    void clear()
+    void clear() noexcept
     {
         data.clear();
         lru.clear();