select: make argument of select_p_select_jobinfo_alloc() consistent

This explicitly declares the argument list of select_p_select_jobinfo_alloc()
to be 'void', to avoid misinterpreting it as an old-style definition:
 * node_select.h already defines select_g_select_jobinfo_alloc(void);
 * the cons_res variant erroneously specified 'select_jobinfo_t *jobinfo';
 * explicit argument list makes bug tracking easier.