tools/nolibc: Add gitignore to avoid git complaints about sysroot

Testing of nolibc can produce a tools/include/nolibc/sysroot file, which
is not known to git. Because it is automatically generated, there is no
reason for it to be known to git. Therefore, add a .gitignore to remove
it from git's field of view.

Signed-off-by: Paul E. McKenney <paulmck@kernel.org>
Cc: Willy Tarreau <w@1wt.eu>
Cc: Ammar Faizi <ammarfaizi2@gnuweeb.org>
Cc: Sven Schnelle <svens@linux.ibm.com>