gst1-validate: fix compile with cairo support

In case cairo support is (auto-)detected cairo PNG support
is required.

Fixes ([1]):

../gst-libs/gst/video/.libs/libgstvalidatevideo-1.0.so: undefined reference to `cairo_surface_write_to_png'
../gst-libs/gst/video/.libs/libgstvalidatevideo-1.0.so: undefined reference to `cairo_image_surface_create_from_png'
collect2: error: ld returned 1 exit status

[1] http://autobuild.buildroot.net/results/c5e/c5eb6cb12dba559226c67f5d258cc151aa8a8d3a



[Peter: reworded comment, dropped extra line]
Signed-off-by: Peter Seiderer <ps.report@gmx.net>
Signed-off-by: Peter Korsgaard <peter@korsgaard.com>