setup.cfg: Add optional gssapi dependency

To install nbxmpp together with the optional dependeny run
`pip install .[gssapi]`.

setup.cfg

@@ -4,7 +4,7 @@ version = 3.0.0-dev2
 description = XMPP Library
 author = Yann Leboulanger, Philipp Hoerist
 author_email = gajim-devel@gajim.org
-long_description = file: README.md  
+long_description = file: README.md
 long_description_content_type = text/markdown
 keywords = xmpp chat
 url = https://dev.gajim.org/gajim/python-nbxmpp
@@ -20,6 +20,9 @@ install_requires =
     precis-i18n>=1.0.0
     idna
 
+[options.extras_require]
+gssapi = gssapi
+
 [options.packages.find]
 exclude =
     test*