Fix unsigned/signed mismatch warning in KTX file reading

On MSCV x86 (32bit) the comparison of qsizetype with
std::numeric_limits<quint32>::max() leads to a warning.
This patch adds a static cast to avoid the warning.

This is an ammendment to 28ecb523ce

Pick-to: 6.2
Change-Id: Ie50572165ac31aafa7d23570bc133c5c96cf8b00
Reviewed-by: Jonas Karlsson <jonas.karlsson@qt.io>
(cherry picked from commit 6463b36da0)
Reviewed-by: Qt Cherry-pick Bot <cherrypick_bot@qt-project.org>
(cherry picked from commit faaef169f5)
(cherry picked from commit 84b2ec6acb)

src/gui/util/qktxhandler.cpp

@@ -105,7 +105,7 @@ QTextureFileData QKtxHandler::read()
         return QTextureFileData();
 
     const QByteArray buf = device()->readAll();
-    if (buf.size() > std::numeric_limits<quint32>::max()) {
+    if (static_cast<size_t>(buf.size()) > std::numeric_limits<quint32>::max()) {
         qWarning(lcQtGuiTextureIO, "Too big KTX file %s", logName().constData());
         return QTextureFileData();
     }