[Metal] Add missing GC.KeeAlive. (#9653)

Do it after the review comments on https://github.com/xamarin/xamarin-macios/pull/9651

src/Metal/MTLIntersectionFunctionTable.cs

@@ -29,6 +29,7 @@ namespace Metal {
 					table.SetBuffers ((IntPtr) buffersPtr, (IntPtr) offsetsPtr, range);
 				}
 			}
+			GC.KeepAlive (buffers);
 		}
 	}
 }