QTemporaryFile: fix narrowing conversion warnings

- Don't use unsigned to avoid negative values (if an unsigned variable
  is assigned a negative value it'll wrap around and become a huge
  positive value, which is usually not what's required)

Change-Id: I5d41280b9ca14c15727f9f2447ed50573b187931
Reviewed-by: Thiago Macieira <thiago.macieira@intel.com>

src/corelib/io/qtemporaryfile.cpp

@@ -45,8 +45,8 @@ QTemporaryFileName::QTemporaryFileName(const QString &templateName)
 {
     // Ensure there is a placeholder mask
     QString qfilename = QDir::fromNativeSeparators(templateName);
-    uint phPos = qfilename.size();
-    uint phLength = 0;
+    qsizetype phPos = qfilename.size();
+    qsizetype phLength = 0;
 
     while (phPos != 0) {
         --phPos;