themes/eOS/meson.build: Install missing PNGs

Follow-up of 4a2a5dc ("eOS theme: Add single button backgrounds")

Without this fix these PNGs are installed when using autotools but not using meson.

Author: bobby285271

src/themes/eOS/meson.build

@@ -18,6 +18,8 @@ install_data(
   'trough_middle_unfocused.png',
   'trough_right.png',
   'trough_right_unfocused.png',
+  'trough_single.png',
+  'trough_single_unfocused.png',
   'unmaximize.png',
   'unmaximize_unfocused_over.png',
   'unmaximize_unfocused.png',