Fix XLSX export failing on PHP 7.1 on systems without "/tmp" path

Since PHP 7.1 tempnam() throws a notice if the dir parameter can't be used : bug #69489 in http://php.net/ChangeLog-7.php#7.1.0

application/xlsxwriter.class.php

@@ -40,7 +40,7 @@ Class XLSXWriter
 	
 	protected function tempFilename()
 	{
-		$filename = tempnam("/tmp", "xlsx_writer_");
+		$filename = tempnam(SetupUtils::GettmpDir(), 'xlsx_writer_');
 		$this->temp_files[] = $filename;
 		return $filename;
 	}