hps_accel/Makefile: add comment about constants
hps_accel/Makefile: add comment about constants

Adds a single line comment to assist readers trying to parse the
additional code that adds Python-defined constants into the C++ build.

Signed-off-by: Alan Green <97e77871b5fcc4941bebceb4338c97c1fd149f8c@google.com>

google/CFU-Playground,google/CFU-Playground,google/CFU-Playground,google/CFU-Playground