tests: fix build dependency
In case of cairo is found but without png support this test will fail during linking becuase the 'cairo_surface_write_to_png' is undefined.
Signed-off-by: Marco Felsch m.felsch@pengutronix.de
In case of cairo is found but without png support this test will fail during linking becuase the 'cairo_surface_write_to_png' is undefined.
Signed-off-by: Marco Felsch m.felsch@pengutronix.de