shared_token_metadata: get_lock is const

The lock is acquired using an a function that
doesn't modify the shared_token_metadata object.

Signed-off-by: Benny Halevy <bhalevy@scylladb.com>

locator/token_metadata.hh

@@ -407,7 +407,7 @@ public:
     // using the schema_tables merge_lock.
     //
     // Must be called on shard 0.
-    future<token_metadata_lock> get_lock() noexcept {
+    future<token_metadata_lock> get_lock() const noexcept {
         return _lock_func();
     }