[GCC] Unreviewed, build fix for Ubuntu 20.04 after 257678@main

Constructors cannot be defaulted in GCC9.3.

* Source/WebCore/css/CSSCustomPropertyValue.h: Define explicit
  constructor for NumericSyntaxValue.

Canonical link: https://commits.webkit.org/257724@main

Source/WebCore/css/CSSCustomPropertyValue.h

@@ -42,7 +42,11 @@ class CSSCustomPropertyValue final : public CSSValue {
         double value;
         CSSUnitType unitType;
 
-        bool operator==(const NumericSyntaxValue&) const = default;
+        bool operator==(const NumericSyntaxValue& other) const
+        {
+            return value == other.value && unitType == other.unitType;
+        }
+
     };
     using SyntaxValue = std::variant<Length, NumericSyntaxValue, StyleColor, RefPtr<StyleImage>, String>;