python: fix tracing after Python 3 refactor

gem5.opt --debug-flags ExecAll

was failing with:

ImportError: No module named defines

on Python 2.

Change-Id: I06959d6e0ccb7c661c7c749af3392632dac779f3
Reviewed-by: Andreas Sandberg <andreas.sandberg@arm.com>
Reviewed-on: https://gem5-review.googlesource.com/c/public/gem5/+/18108
Maintainer: Andreas Sandberg <andreas.sandberg@arm.com>
Tested-by: kokoro <noreply+kokoro@google.com>

src/python/m5/main.py

@@ -193,7 +193,7 @@ def interact(scope):
 
 
 def _check_tracing():
-    import defines
+    from . import defines
 
     if defines.TRACING_ON:
         return