selftests: Fix the executable permissions for fib_tests.sh

Commit 04a331c9 reverted change 2537b637 (ipv4: Fix
incorrect route flushing when source address is deleted) that
added the fib_tests.sh file back with wrong permissions.
Fix that in this commit.

Signed-off-by: Rishabh Bhatnagar <risbhat@amazon.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>