mark ftp_close as impure

in theory, all ftp_* functions are impure, but not marking them
encourages the good practice of checking the return value.

However, ftp_close(), while it's possible for it to fail, what good
would checking the return type do? How is one supposed to handle the
failure to close a stream in a way that affects future program flow?

src/Psalm/Internal/Codebase/Functions.php

@@ -367,6 +367,9 @@ class Functions
             'mysqli_select_db', 'mysqli_dump_debug_info', 'mysqli_kill', 'mysqli_multi_query',
             'mysqli_next_result', 'mysqli_options', 'mysqli_ping', 'mysqli_query', 'mysqli_report',
             'mysqli_rollback', 'mysqli_savepoint', 'mysqli_set_charset', 'mysqli_ssl_set',
+
+            // ftp
+            'ftp_close',
         ];
 
         if (\in_array(strtolower($function_id), $impure_functions, true)) {