Renaming getip() is one way of fixing it and it should work this way. What bothers me is the fact that cforms actually looks out for an existing getip() function and would not attempt to redeclare it, if found:

if ( !function_exists('getip') ) :

Can it be, that the error message stems from the other plugin? In any regard, your solution is also correct and should do the trick.

Once activated, you see no top level menu item 'cforms' (It's not under Options anymore!) ? No error messages when activating it? – not that I would expect them ;)