/usr/include/GNUstep/RSSKit is in librsskit-dev 0.4-1.
This file is owned by root:root, with mode 0o777.
It is a symlink to /usr/include/GNUstep/../../lib/GNUstep/Frameworks/RSSKit.framework/Headers
/usr/include/GNUstep/../../lib/GNUstep/Frameworks/RSSKit.framework/Headers