configure: Fix compile warning in PNG test

Fix compile warning (variable 'png_ptr' set but not used) in the
PNG detection test code.

Signed-off-by: Peter Maydell <peter.maydell@linaro.org>
Reviewed-by: Stefan Weil <sw@weilnetz.de>
Signed-off-by: Blue Swirl <blauwirbel@gmail.com>