Replace gp_endswith() with working implementation

In [22349] we introduced a call to gp_endswith(),
whcih doesn't exist in WordPress, only in GlotPress.

See #22337


git-svn-id: http://core.svn.wordpress.org/trunk@22350 1a063a9b-81f0-0310-95a4-ce76da25c4cd

wp-includes/pomo/po.php

@@ -355,7 +355,7 @@ class PO extends Gettext_Translations {
 			return true;
 		}
 		$line = $use_last_line? $last_line : fgets($f);
-		$line = gp_endswith( $line, "\r\n" )? rtrim( $line, "\r\n" ) . "\n" : $line;
+		$line = ( "\r\n" == substr( $line, -2 ) ) ? rtrim( $line, "\r\n" ) . "\n" : $line;
 		$last_line = $line;
 		$use_last_line = false;
 		return $line;