Fixed a "NameError uninitialized constant" error.

On startup of msfconsole, the following error occurred:

  modules/exploits/freebsd/local/mmap.rb: NameError uninitialized constant Msf::Post::Common

The addition of a corresponding 'require' line removed that error.

Signed-off-by: Thorsten Fischer <thorsten@froschi.org>
Land #2076, missing require in exploit/freebsd/mmap

[Closes #2076]