Updated declaration of GLFWcursorposfun to latest version of glfw3

In the latest version of glfw3, GLFWcursorposfun gets x and y as doubles:
https://github.com/elmindreda/glfw/blob/master/include/GL/glfw3.h#L691

Yet Derelict3 accepts them as ints:
alias void function(GLFWwindow*, int, int) GLFWcursorposfun;

This simply fixes the alias.
Running this test with the fixed alias gives correct results: https://gist.github.com/nguillemot/5411073