linux: make sure ExecutionError is defined

ExecutionError is used all over the place in this file, but python is
not happy about it:

    >       except ExecutionError:
    E       NameError: name 'ExecutionError' is not defined

    labgridhelper/labgridhelper/linux.py:173: NameError

Fixes: fcdeab4 (2019-02-27, "linux: add helpers from InfoDriver")
Signed-off-by: Roland Hieber <[email protected]>

Author: rohieb

labgridhelper/linux.py

@@ -1,4 +1,6 @@
 from labgrid.protocol import CommandProtocol
+from labgrid.driver import ExecutionError
+
 import json
 import re