scons: Make env['USE_PYTHON'] a source for createEnumStrings.

scons: Make env['USE_PYTHON'] a source for createEnumStrings.

That clues scons in to the fact that the contents of the target of that
function depends on the value of that construction variable.

Change-Id: I803322ea1a178152da3d95dffffe20acd4271c88
Reviewed-on: https://gem5-review.googlesource.com/3083
Reviewed-by: Jason Lowe-Power <jason@lowepower.com>
Reviewed-by: Paul Rosenfeld <prosenfeld@micron.com>
Reviewed-by: Andreas Sandberg <andreas.sandberg@arm.com>
Maintainer: Andreas Sandberg <andreas.sandberg@arm.com>