Change an unsigned integer to a signed integer in profilewidget2.cpp,
because commit 1f8506c changed the definition of duration_t from
unsigned to signed.
Since this value represents the number of seconds since a dive started,
there is no possible chance of overflow problems.
Signed-off-by: Berthold Stoeger <bstoeger@mail.tuwien.ac.at>
f8fcd65bc4