QWindowsPipeWriter: Fix developer build with MinGW.

Fix signedness in comparion:
io\qwindowspipewriter.cpp: In member function 'void QWindowsPipeWriter::notified(DWORD, DWORD)':
io\qwindowspipewriter.cpp:134:65: error: comparison between signed and unsigned integer expressions [-Werror=sign-compare]
     Q_ASSERT(errorCode != ERROR_SUCCESS || numberOfBytesWritten == buffer.size());

Amends change a4d26cf5

.

Task-number: QTBUG-52401
Change-Id: If0c0e2107342408675fa00b93f28c9de339080f6
Reviewed-by: Joerg Bornemann <joerg.bornemann@theqtcompany.com>

src/corelib/io/qwindowspipewriter.cpp

@@ -131,7 +131,7 @@ void QWindowsPipeWriter::notified(DWORD errorCode, DWORD numberOfBytesWritten)
     notifiedCalled = true;
     writeSequenceStarted = false;
     numberOfBytesToWrite = 0;
-    Q_ASSERT(errorCode != ERROR_SUCCESS || numberOfBytesWritten == buffer.size());
+    Q_ASSERT(errorCode != ERROR_SUCCESS || numberOfBytesWritten == DWORD(buffer.size()));
     buffer.clear();
 
     switch (errorCode) {