linuxbuild: fix NameError that 'os' is not defined

Signed-off-by: NWei Jiangang <weijg.fnst@cn.fujitsu.com>

examples/tests/linuxbuild.py

 #!/usr/bin/env python
 
+import os
+
 from avocado import Test
 from avocado import main
 from avocado.utils import kernel