HPack: make padding_is_valid static constexpr

... because we can.

Change-Id: I3a813fa9c33e47d20a00e6a331cbbcf6f85e63e7
Reviewed-by: Mårten Nordheim <marten.nordheim@qt.io>
Reviewed-by: Timur Pocheptsov <timur.pocheptsov@qt.io>

src/network/access/http2/huffman.cpp

@@ -402,6 +402,7 @@ void huffman_encode_string(const QByteArray &inputData, BitOStream &outputStream
         outputStream.writeBits(0xff, 8 - outputStream.bitLength() % 8);
 }
 
+static constexpr
 bool padding_is_valid(quint32 chunk, quint32 nBits)
 {
     Q_ASSERT(nBits);