I can't give you a clear cut answer to your question, but I'd like to point out a few of the considerations that pertains to it and show how you could argue for and against.
To begin, let's start by looking at responsibility in terms of unhealthy foods. Alanf in the currently top voted comment takes the individualist stance that your colleagues are responsible for their own actions and that you can't be blamed for their bad choices. This is a valid ethical stance, but from the other end of the spectrum you could equally well argue the opposite.
Imagine if you have a colleague who is trying to lose weight but struggling with dieting sitting next to the pantry, and you have 20 boxes of twinkies that you would like to get rid off. While Alanf would argue that you are within your right to deposit the twinkies in the pantry, somebody else would probably argue that you would put your colleague in a situation that would make it harder for them to make the right choice, and in this sense being responsible for the twinkies they eat.
It's worth considering if you find yourself somewhat obliged to help other people eat healthy or if you think of it strictly as a personal matter. Both stances can be valid depending on the reasons that inform your own choices. Say you are eating healthy because of a societal concern that obesity is costly to society in terms of medical expenditure. In this case your motivation for eating healthy are not tied to your own consumption, but for society as a whole, and it would be counter productive to provide other people with unhealthy food. On the other hand you might have found a diet that makes you feel healthy in which case your own choices have nothing to do with those of your colleagues.
What it boils down to, is a discussion between your responsibilities towards other people versus your respect for other people's individual choices.
As a society we try to outlaw crack cocain, and most here would probably agree that it would be unethical to put crack in the pantry for common use. At the same time you might be convinced that wearing blue on Tuesdays causes cancer but most people would be upset if you started telling them what color clothes to wear. At some point we collectively draw a line between what we can impose on others and what they have the right to decide for themselves. Putting unhealthy food in the pantry lies somewhere close to that line, but how we decide which side it lies on depends on our outlook on society.
A libertarian would probably move the line as far towards individual responsibility as possible and would be convinced that there was nothing ethically wrong with offering your coworkers unhealthy food. A socialist (in lack of a better term) would most likely argue that our interests as a society is likely to trump the free will of the individuals in it.
I hope this helps