Fix EditorImportPlugin::get_preset_count

Was missing a const...

editor/import/editor_import_plugin.cpp

@@ -56,7 +56,7 @@ String EditorImportPlugin::get_preset_name(int p_idx) const {
 	return get_script_instance()->call("get_preset_name", p_idx);
 }
 
-int EditorImportPlugin::get_preset_count() {
+int EditorImportPlugin::get_preset_count() const {
 	ERR_FAIL_COND_V(!(get_script_instance() && get_script_instance()->has_method("get_preset_count")), 0);
 	return get_script_instance()->call("get_preset_count");
 }

editor/import/editor_import_plugin.h

@@ -43,7 +43,7 @@ public:
 	virtual String get_visible_name() const;
 	virtual void get_recognized_extensions(List<String> *p_extensions) const;
 	virtual String get_preset_name(int p_idx) const;
-	virtual int get_preset_count();
+	virtual int get_preset_count() const;
 	virtual String get_save_extension() const;
 	virtual String get_resource_type() const;
 	virtual void get_import_options(List<ImportOption> *r_options, int p_preset) const;