Fix -Wint-conversion error with gcc 14.

```
screen.c: In function 'screen_get_sdir':
screen.c:49:21: error: passing argument 2 of 'g_strlcpy' makes pointer from integer without a cast [-Wint-conversion]
   49 |     g_strlcpy(sdir, sizeof(sdir), getenv("SCREENDIR"));
      |                     ^~~~~~~~~~~~
      |                     |
      |                     long unsigned int
In file included from /usr/include/glib-2.0/glib/gstring.h:37,
                 from /usr/include/glib-2.0/glib/giochannel.h:36,
                 from /usr/include/glib-2.0/glib.h:56,
                 from screen.c:29:
/usr/include/glib-2.0/glib/gstrfuncs.h:124:55: note: expected 'const gchar *' {aka 'const char *'} but argument is of type 'long unsigned int'
  124 |                                         const gchar  *src,
      |                                         ~~~~~~~~~~~~~~^~~
screen.c:49:35: error: passing argument 3 of 'g_strlcpy' makes integer from pointer without a cast [-Wint-conversion]
   49 |     g_strlcpy(sdir, sizeof(sdir), getenv("SCREENDIR"));
      |                                   ^~~~~~~~~~~~~~~~~~~
      |                                   |
      |                                   char *
/usr/include/glib-2.0/glib/gstrfuncs.h:125:55: note: expected 'gsize' {aka 'long unsigned int'} but argument is of type 'char *'
  125 |                                         gsize         dest_size);
      |                                         ~~~~~~~~~~~~~~^~~~~~~~~
```

--- apt-dater-1.0.4-origin/src/screen.c
+++ apt-dater-1.0.4/src/screen.c
@@ -46,7 +46,7 @@
   static gchar sdir[PATH_MAX];
 
   if (g_strcmp0(getenv("SCREENDIR"), NULL) != 0) {
-    g_strlcpy(sdir, sizeof(sdir), getenv("SCREENDIR"));
+    g_strlcpy(sdir, getenv("SCREENDIR"), sizeof(sdir));
     return sdir;
   }