Use correct namespace for checking if the isDefaultRootsPemSet method exists. (#31580)

The `method_exists` function requires a fully qualified class name to be
sent to check if a method exists. The current class was missing the
namespace, which means the function always returns `false`. In our
application this caused the credentials to be loaded many times over,
which ate up some CPU. This bug fix ensures that this is only run once
per request.

src/php/lib/Grpc/BaseStub.php

@@ -43,7 +43,7 @@ class BaseStub
      */
     public function __construct($hostname, $opts, $channel = null)
     {
-        if (!method_exists('ChannelCredentials', 'isDefaultRootsPemSet') ||
+        if (!method_exists('Grpc\ChannelCredentials', 'isDefaultRootsPemSet') ||
             !ChannelCredentials::isDefaultRootsPemSet()) {
             $ssl_roots = file_get_contents(
                 dirname(__FILE__).'/../../../../etc/roots.pem'