base: Fix uninitialized variable in Flag

base: Fix uninitialized variable in Flag

This was uninitialized, and was breaking expected values
under certain situations.

Change-Id: If51ab6ae038c7c397bc83de1c73af348c1db4ef8
Signed-off-by: Daniel R. Carvalho <odanrc@yahoo.com.br>
Reviewed-on: https://gem5-review.googlesource.com/c/public/gem5/+/38707
Reviewed-by: Bobby R. Bruce <bbruce@ucdavis.edu>
Reviewed-by: Andreas Sandberg <andreas.sandberg@arm.com>
Maintainer: Bobby R. Bruce <bbruce@ucdavis.edu>
Tested-by: kokoro <noreply+kokoro@google.com>