clockfix: Switch comp_timeb() and add_ms_to_timeb_diff() to return int64_t.

This prevents overflows because int32_t can't hold much time in milliseconds.

git-svn-id: http://streamboard.de.vu/svn/oscam/trunk@9377 4b0bc96b-bc66-0410-9d44-ebda105a78c1