Fix build against Qt 5.15.3

Use QLatin1Char to prevent build error due to stricter QT_NO_CAST_FROM_ASCII

Signed-off-by: Nicolas Fella <nicolas.fella@gmx.de>

src/csync/csync_exclude.cpp

@@ -588,7 +588,7 @@ QString ExcludedFiles::convertToRegexpSyntax(QString exclude, bool wildcardsMatc
             // Translate [! to [^
             QString bracketExpr = exclude.mid(i, j - i + 1);
             if (bracketExpr.startsWith(QLatin1String("[!")))
-                bracketExpr[1] = '^';
+                bracketExpr[1] = QLatin1Char('^');
             regex.append(bracketExpr);
             i = j;
             break;