fix(grid): buttons style in ExcelStyleFiltering in IE11

Author: SisIvanova

projects/igniteui-angular/src/lib/core/styles/components/button-group/_button-group-theme.scss

@@ -195,7 +195,7 @@
         background: --var($theme, 'item-background');
         min-width: $group-item-min-width;
         display: flex;
-        flex: 1 0 0%;
+        flex: 1 0 auto;
         justify-content: center;
         align-items: center;
         text-decoration: none;