fix(analysis/locally_convex/balanced_hull_core): minimize import (#13450

)

I'm doing this because I need to have `balanced_hull_core` before `normed_space.finite_dimensional` and this little change seems to be enough for that, but I think at some point we'll need to move lemmas so that normed_spaces come as late as possible

src/analysis/locally_convex/balanced_core_hull.lean

@@ -3,7 +3,7 @@ Copyright (c) 2022 Moritz Doll. All rights reserved.
 Released under Apache 2.0 license as described in the file LICENSE.
 Authors: Moritz Doll
 -/
-import analysis.seminorm
+import analysis.locally_convex.basic
 import order.closure
 
 /-!