Media Grid: fix z-index of attachment "check"

See #24716.

Built from https://develop.svn.wordpress.org/trunk@29084


git-svn-id: http://core.svn.wordpress.org/trunk@28870 1a063a9b-81f0-0310-95a4-ce76da25c4cd

wp-includes/css/media-views-rtl.css

@@ -880,6 +880,7 @@
 	height: 24px;
 	width: 24px;
 	position: absolute;
+	z-index: 10;
 	top: -6px;
 	left: -6px;
 	outline: none;

wp-includes/css/media-views.css

@@ -880,6 +880,7 @@
 	height: 24px;
 	width: 24px;
 	position: absolute;
+	z-index: 10;
 	top: -6px;
 	right: -6px;
 	outline: none;