LibWeb: CSSImportRule::set_style_sheet() should take a CSSStyleSheet

Spotted by @tomuta in #6086.

Userland/Libraries/LibWeb/CSS/CSSImportRule.h

@@ -48,7 +48,7 @@ public:
     bool has_import_result() const { return !m_style_sheet.is_null(); }
     RefPtr<CSSStyleSheet> loaded_style_sheet() { return m_style_sheet; }
     const RefPtr<CSSStyleSheet> loaded_style_sheet() const { return m_style_sheet; }
-    void set_style_sheet(const RefPtr<StyleSheet>& style_sheet) { m_style_sheet = style_sheet; }
+    void set_style_sheet(const RefPtr<CSSStyleSheet>& style_sheet) { m_style_sheet = style_sheet; }
 
     virtual StringView class_name() const { return "CSSImportRule"; };
     virtual Type type() const { return Type::Import; };