clang wants this to be static now

Author: MjnMixael

code/lab/dialogs/lab_ui.cpp

@@ -206,7 +206,7 @@ void LabUi::build_prop_list()
 	}
 }
 
-void LabUi::build_background_list() const
+void LabUi::build_background_list()
 {
 	SCP_vector<SCP_string> t_missions;
 

code/lab/dialogs/lab_ui.h

@@ -34,7 +34,7 @@ class LabUi {
 	static void build_debris_list();
 	static void build_prop_list();
 	static void build_prop_subtype_list();
-	void build_background_list() const;
+	static void build_background_list();
 	void show_render_options();
 	void show_object_options() const;
 	void show_object_selector() const;