/usr/include/ is in ruby2.1-dev 2.1.5-2+deb8u3.
This file is owned by root:root, with mode 0o755.
..
/usr/include/x86_64-linux-gnu/
/usr/include/x86_64-linux-gnu/ruby-2.1.0/
/usr/include/x86_64-linux-gnu/ruby-2.1.0/ruby/
/usr/include/x86_64-linux-gnu/ruby-2.1.0/ruby/config.h
/usr/include/ruby-2.1.0/
/usr/include/ruby-2.1.0/ruby.h
/usr/include/ruby-2.1.0/ruby/
/usr/include/ruby-2.1.0/ruby/version.h
/usr/include/ruby-2.1.0/ruby/ruby.h
/usr/include/ruby-2.1.0/ruby/io.h
/usr/include/ruby-2.1.0/ruby/oniguruma.h
/usr/include/ruby-2.1.0/ruby/intern.h
/usr/include/ruby-2.1.0/ruby/encoding.h
/usr/include/ruby-2.1.0/ruby/thread.h
/usr/include/ruby-2.1.0/ruby/regex.h
/usr/include/ruby-2.1.0/ruby/util.h
/usr/include/ruby-2.1.0/ruby/subst.h
/usr/include/ruby-2.1.0/ruby/re.h
/usr/include/ruby-2.1.0/ruby/vm.h
/usr/include/ruby-2.1.0/ruby/dl.h
/usr/include/ruby-2.1.0/ruby/missing.h
/usr/include/ruby-2.1.0/ruby/defines.h
/usr/include/ruby-2.1.0/ruby/backward/
/usr/include/ruby-2.1.0/ruby/backward/rubysig.h
/usr/include/ruby-2.1.0/ruby/backward/util.h
/usr/include/ruby-2.1.0/ruby/backward/rubyio.h
/usr/include/ruby-2.1.0/ruby/backward/classext.h
/usr/include/ruby-2.1.0/ruby/backward/st.h
/usr/include/ruby-2.1.0/ruby/debug.h
/usr/include/ruby-2.1.0/ruby/st.h
/usr/include/ruby-2.1.0/ruby/digest.h