Had to edit Rakefile to compile on Linux

I was getting bugs every time 'rbSFML.hpp' was included, to fix it I had to edit the ext folder on the Rakefile to explicitly show that it was on the current folder.

I know that is just a 'workaround' (that works), but I find important to show it to you guys.

Maybe it run OK on Windows & OSX without this quick fix, but this was the only way I find to compile it on Linux (64x_86x, Fedora 18, RVM Ruby 1.9.3)