Consider to move mkmf.rb to ruby-devel

[voxik]

Consider to move mkmf.rb to ruby-devel:

[root@localhost /]# rpm -q ruby
ruby-1.9.3.0-2.fc17.i686
[root@localhost /]# rpm -q ruby-devel
package ruby-devel is not installed
[root@localhost /]# ruby -e "require 'mkmf'"
mkmf.rb can't find header files for ruby at /usr/share/include/ruby.h