avutil/opencl: don't include config.h

It's not an installed header.

Tested-by: Thilo Borgmann <thilo.borgmann@mail.de>
Tested-by: Wei Gao <highgod0401@gmail.com>
Reviewed-by: Michael Niedermayer <michaelni@gmx.at>
Signed-off-by: James Almer <jamrial@gmail.com>