Ensure void in empty parameter list

According to ISO/IEC 9899:2011, section 6.7.6.3, "Function declarators
(including prototypes)": "The special case of an unnamed parameter of
type void as the only item in the list specifies that the function has
no parameters."

Signed-off-by: Guilherme Maciel Ferreira <guilherme.maciel.ferreira@gmail.com>