configure: Keep LIBPNG_CFLAGS settings if png.h is available.

[Piotr Caban]

It fixes #ifdef HAVE_PNG_H checks if library is not available.

Signed-off-by: Piotr Caban <piotr(a)codeweavers.com>
---
  configure.ac | 2 +-
  1 file changed, 1 insertion(+), 1 deletion(-)