python: Fix broken call to m5.fatal in _check_tracing()

The call to m5.fatal in _check_tracing() fails because it has not been
imported at this point.

Change-Id: I60b1de6128d0ffc29e03e9ed98a8f9f679ef0ff9
Reviewed-on: https://gem5-review.googlesource.com/c/public/gem5/+/50447
Maintainer: Bobby Bruce <bbruce@ucdavis.edu>
Reviewed-by: Bobby Bruce <bbruce@ucdavis.edu>
Tested-by: kokoro <noreply+kokoro@google.com>

src/python/m5/main.py

@@ -334,12 +334,13 @@ def interact(scope):
 
 
 def _check_tracing():
+    import m5
     import _m5.core
 
     if _m5.core.TRACING_ON:
         return
 
-    fatal("Tracing is not enabled.  Compile with TRACING_ON")
+    m5.fatal("Tracing is not enabled.  Compile with TRACING_ON")
 
 
 def main():