backend: fix build against upcoming `gcc-14` (`-Werror=calloc-transposed-args`)

`gcc-14` added a new `-Wcalloc-transposed-args` warning recently. It
detected minor infelicity in `calloc()` API usage in `wlroots`:

    ../backend/libinput/tablet_pad.c: In function 'add_pad_group_from_libinput':
    ../backend/libinput/tablet_pad.c:36:38: error: 'calloc' sizes specified with 'sizeof' in the earlier argument and not in the later argument [-Werror=calloc-transposed-args]
       36 |         group->rings = calloc(sizeof(unsigned int), group->ring_count);
          |                                      ^~~~~~~~
    ../backend/libinput/tablet_pad.c:36:38: note: earlier argument should specify number of elements, later size of each element

backend/libinput/tablet_pad.c

@@ -33,7 +33,7 @@ static void add_pad_group_from_libinput(struct wlr_tablet_pad *pad,
 			++group->ring_count;
 		}
 	}
-	group->rings = calloc(sizeof(unsigned int), group->ring_count);
+	group->rings = calloc(group->ring_count, sizeof(unsigned int));
 	if (group->rings == NULL) {
 		goto group_fail;
 	}
@@ -50,7 +50,7 @@ static void add_pad_group_from_libinput(struct wlr_tablet_pad *pad,
 			++group->strip_count;
 		}
 	}
-	group->strips = calloc(sizeof(unsigned int), group->strip_count);
+	group->strips = calloc(group->strip_count, sizeof(unsigned int));
 	if (group->strips == NULL) {
 		goto group_fail;
 	}
@@ -66,7 +66,7 @@ static void add_pad_group_from_libinput(struct wlr_tablet_pad *pad,
 			++group->button_count;
 		}
 	}
-	group->buttons = calloc(sizeof(unsigned int), group->button_count);
+	group->buttons = calloc(group->button_count, sizeof(unsigned int));
 	if (group->buttons == NULL) {
 		goto group_fail;
 	}