drivers: neural_net: Use correct Kconfig variable

The Kconfig variable for selecting neural network accelerator driver(s)
was changed from NEURAL_NET to NEURAL_NET_ACCEL
The CMakeLists.txt however, was not updated to use the updated variable
This commit updates CMakeLists.txt to include neural_net if
CONFIG_NEURAL_NET_ACCEL is defined.

Signed-off-by: Sathish Kuttan <sathish.k.kuttan@intel.com>