Can't see it very well, at all, in this picture. Look closer. If the rust appears to be surface rust (which is by far the most likely) then you're fine. You should worry more if the paint appears cracked from bending or impact, anything which might indicate that the head tube is separating from the fork blades. But a bit of rust around the joint, by itself, isn't an indicator of danger.
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"c" is not a unit that measures tire width