Fix build on gentoo linux with gcc 4.6

Hi Zini

This patch fixes following error with gcc 4.6 on gentoo linux. Please pull, if you agree with the fix.

/var/tmp/portage/games-rpg/openmw-9999/work/openmw-9999/apps/openmw/mwgui/dialogue.cpp:
In constructor ‘MWGui::DialogueWindow::DialogueWindow()’:
/var/tmp/portage/games-rpg/openmw-9999/work/openmw-9999/apps/openmw/mwgui/dialogue.cpp:271:44:
error: ‘bind’ is not a member of ‘boost’

Thanks
edmondo