Reader/Gnumeric Failure with PHP8 (PHPOffice#1662)

* Reader/Gnumeric Failure with PHP8

An explicit cast from SimpleXML to int is now needed with PHP8,
where it was performed implicitly with PHP7.
Unit tests run correctly for both PHP7 and PHP8 on corrected code.

src/PhpSpreadsheet/Reader/Gnumeric.php

@@ -543,7 +543,8 @@ public function loadIntoExisting(string $pFilename, Spreadsheet $spreadsheet): S
 
                     $endColumn = ($styleAttributes['endCol'] > $maxCol) ? $maxCol : (int) $styleAttributes['endCol'];
                     $endColumn = Coordinate::stringFromColumnIndex($endColumn + 1);
-                    $endRow = 1 + (($styleAttributes['endRow'] > $maxRow) ? $maxRow : $styleAttributes['endRow']);
+
+                    $endRow = 1 + (($styleAttributes['endRow'] > $maxRow) ? $maxRow : (int) $styleAttributes['endRow']);
                     $cellRange = $startColumn . $startRow . ':' . $endColumn . $endRow;
 
                     $styleAttributes = $styleRegion->Style->attributes();