yes, you can include Expat (MIT) licensed libraries in your GPL plugin. WordPress plugin guidelines states,
Although any GPL-compatible license is acceptable, using the same license as WordPress — “GPLv2 or later” — is strongly recommended. All code, data, and images — anything stored in the plugin directory hosted on WordPress.org — must comply with the GPL or a GPL-Compatible license. Included third-party libraries, code, images, or otherwise, must be compatible.
Here is a list of licenses compatible with GPL.