Fix check for when only flowers remain

The game is supposed to be won automatically when there is only tiles
with flowers left even if there are no flags there. The check for this
does not take the number of flags into account, which means that it
won't work if some of tiles are marked with flags. Fix the check by
subtracting the number of flagged tiles.

Signed-off-by: Marcus Karlsson <mk@acc.umu.se>