Rename requireGcStore to GcStore::require

I should have done this to begin with. This will be nicer once more
Store sub-interfaces exist too, to illustrate the pattern.

src/libstore/gc-store.hh

@@ -77,8 +77,8 @@ struct GcStore : public virtual Store
 
     /* Perform a garbage collection. */
     virtual void collectGarbage(const GCOptions & options, GCResults & results) = 0;
+
+    static GcStore & require(Store & store);
 };
 
-GcStore & requireGcStore(Store & store);
-
 }