Remove unnecessary #includes from the kqueue(2) man page.

[ed]

Summary

Now that <sys/event.h> can be included on its own, adjust the manual
page accordingly. Remove both unnecessary #include statements from the
synopsis and the example code.

While there, also add a note to the BUGS section to mention that
previous versions of this header file still depend on <sys/types.h>.

[ngie]

Thank you very much ed@ :)!