win: constify term_preedit_str_get()

src/bin/win.c

@@ -1165,7 +1165,7 @@ _cb_win_key_up(void *data,
    keyin_handle_up(&wn->khdl, ev);
 }
 
-char *
+const char *
 term_preedit_str_get(Term *term)
 {
    Win *wn = term->wn;

src/bin/win.h

@@ -66,7 +66,7 @@ void term_down(Term *term);
 void term_left(Term *term);
 void term_right(Term *term);
 
-char *
+const char *
 term_preedit_str_get(Term *term);
 
 void win_font_size_set(Win *wn, int new_size);