build: libweston doesn't need -export-dynamic

According to https://gitlab.freedesktop.org/wayland/weston/merge_requests/159#note_148104


it doesn't make sense to use export-dynamic on libraries.

Signed-off-by: Marius Vlad <marius.vlad@collabora.com>
Reported-by: Pekka Paalanen <pekka.paalanen@collabora.com>