Fix non-included header in config.c.

When compiling as experimental, WEBP_EXPERIMENTAL_FEATURES
would not be defined because the header defining it would
not be included.
Hence runtime errors in debug mode when running:
./cwebp -lossles whatever
...
Error! Cannot encode picture as WebP
Error code: 4 (INVALID_CONFIGURATION: configuration is invalid)

(detail: WebPConfig would have a random value set for
delta_palettization as config.c does not consider
it to exist.)

Change-Id: I41761cffe81a971130ed514b195a73d1c6dac1b7
1 file changed