meson: fix building screen-share module

When building screen-share module with meson loading the module
fails with:
  [00:01:28.604] Failed to load module: /usr/local/lib/weston/screen-share.so: undefined symbol: os_create_anonymous_file
  Failed to process Wayland connection: Broken pipe
  failed to create display: Broken pipe

The function os_create_anonymous_file is defined in libshared,
adding libshared to the dependency makes sure the function gets
compiled into screen-share.so.

Fixes: https://gitlab.freedesktop.org/wayland/weston/issues/208



Signed-off-by: Stefan Agner <stefan@agner.ch>