patch 9.0.1336: functions without arguments are not always declared properly

Problem:    Functions without arguments are not always declared properly.
Solution:   Use "(void)" instead of "()". (Yegappan Lakshmanan, closes #12031)

src/charset.c

@@ -1764,7 +1764,7 @@ skipwhite_and_nl(char_u *q)
  * columns (bytes) at the start of a given line
  */
     int
-getwhitecols_curline()
+getwhitecols_curline(void)
 {
     return getwhitecols(ml_get_curline());
 }