surface-screenshot-test.c uses file_create_dated() provided by libshared, so it
needs to link libshared.

This was not a problem when unresolved symbols during build were allowed and
the symbols was provided by the weston executable which accidentally exported
all libshared symbols. This would become a problem when we disallow unresolved
symbols project-wide, so fix it.

Signed-off-by: Pekka Paalanen <pekka.paalanen@collabora.com>