ex_cmds*: Silence -Wstrict-prototypes warnings

Due to a number of places where headers generated from these files are included
gcc with -DEXITFREE produces *loads* of warnings for these functions.

src/nvim/ex_cmds.c

@@ -6135,7 +6135,7 @@ char_u * sign_typenr2name(int typenr)
 /*
  * Undefine/free all signs.
  */
-void free_signs()
+void free_signs(void)
 {
   while (first_sign != NULL)
     sign_undefine(first_sign, NULL);