rcutorture: Rename TREE_RCU-Kconfig.txt

It used to be that:

    git ls-files "*Kconfig*"

would find all Kconfig files and would only find Kconfig files. This
commit renames TREE_RCU-Kconfig.txt to TREE_RCU-kconfig.txt so that this
is once again true.

Signed-off-by: Paul Bolle <pebolle@tiscali.nl>
Signed-off-by: Paul E. McKenney <paulmck@linux.vnet.ibm.com>
Reviewed-by: Josh Triplett <josh@joshtriplett.org>