python: Add missing defines import

python: Add missing defines import

The _check_tracing helper function in main.py depends on defines to
check if tracing has been enabled at compile time. This module is
imported in main() but not at the module level, which breaks this
function.

Change-Id: I26d65a4320da8618e0e552553695884fd2c880e0
Signed-off-by: Andreas Sandberg <andreas.sandberg@arm.com>
Reviewed-on: https://gem5-review.googlesource.com/c/16402
Reviewed-by: Giacomo Travaglini <giacomo.travaglini@arm.com>