selftests/net: Fix permissions for fib_tests.sh

fib_tests.sh became non-executable at some point. This is
what happens:
  selftests: net: fib_tests.sh: Warning: file fib_tests.sh is
  not executable, correct this.
  not ok 1..11 selftests: net: fib_tests.sh [FAIL]

Fixes: d69faad7 ("selftests: fib_tests: Add prefix route tests with metric")
Signed-off-by: Daniel Díaz <daniel.diaz@linaro.org>
Signed-off-by: David S. Miller <davem@davemloft.net>