fix(grid): buttons style in ExcelStyleFiltering in IE11

Author: SisIvanova

projects/igniteui-angular/src/lib/core/styles/components/grid/_excel-filtering-theme.scss

@@ -407,6 +407,10 @@
             }
         }
 
+        igx-buttongroup {
+            min-width: 30%;
+        }
+
         [igxButton='icon'] {
             margin-#{$left}: rem(16px);
         }