syntax: add const to get_syntax_attr() params

src/nvim/screen.c

@@ -3294,8 +3294,7 @@ win_line (
           did_emsg = FALSE;
 
           syntax_attr = get_syntax_attr((colnr_T)v - 1,
-              has_spell ? &can_spell :
-              NULL, FALSE);
+                                        has_spell ? &can_spell : NULL, false);
 
           if (did_emsg) {
             wp->w_s->b_syn_error = TRUE;

src/nvim/syntax.c

@@ -1568,9 +1568,9 @@ syn_finish_line(
  */
 int
 get_syntax_attr(
-    colnr_T col,
-    bool *can_spell,
-    int keep_state                 /* keep state of char at "col" */
+    const colnr_T col,
+    bool *const can_spell,
+    const bool keep_state           // keep state of char at "col"
 )
 {
   int attr = 0;