QCss: Fix parsing of charset

When including a CSS file in a HTML file sent to QTextDocument, and the
CSS file starts with «@charset "UTF-8";», which is the correct way of
declaring that, the parsing fails.
If you omit the space, like «@charset"UTF-8";» the parsing succeeds,
which is wrong.

Fix this by expecting and swallowing whitespace after the @charset
tokens.

Task-number: QTBUG-54829
Change-Id: I32044e8d24bda70c1eb06bf74af50d4cabe2211d
Reviewed-by: Allan Sandfeld Jensen <allan.jensen@qt.io>

src/gui/text/qcssparser.cpp

@@ -2175,6 +2175,7 @@ void Parser::init(const QString &css, bool isFile)
 bool Parser::parse(StyleSheet *styleSheet, Qt::CaseSensitivity nameCaseSensitivity)
 {
     if (testTokenAndEndsWith(ATKEYWORD_SYM, QLatin1String("charset"))) {
+        while (test(S) || test(CDO) || test(CDC)) {}
         if (!next(STRING)) return false;
         if (!next(SEMICOLON)) return false;
     }