The regular expression is fine, you just need to highlight the first group in the match, not the whole of it. Replace . 'font-lock-function-name-face
with 1 'font-lock-function-name-face
.
Another thing to change, just a recommendation, is that font-lock-add-keywords
accepts the mode name as the first argument. So you don't need to use the hook.
Result:
(font-lock-add-keywords
'c-mode
'(("\\<\\(\\sw+\\) ?(" 1 'font-lock-function-name-face)))