i18n: Update POTFILES.in

[yakushabb]

• Add missing files to extract some strings.

src/bz-screenshot-page.c: 1
408: 26:   toast = adw_toast_new (_ ("Copied!"));

src/bz-screenshots-carousel.c: 1
287: 30:   caption = g_strdup_printf (_ ("Screenshot %u of %u"), index + 1, n_items);

src/bz-subcategory-list.c: 3
 78: 44:           AdwToast *toast = adw_toast_new (_ ("No Results Found"));
 89: 47:           subtitle         = g_strdup_printf (_ ("%d Applications"), n_items);
100: 40:       AdwToast *toast = adw_toast_new (_ ("Search failed"));

[kolunmi]

Thank you!