debian: fix network on Wheezy

source-directory ifupdown directive has been introduced in version
0.7.44. Wheezy comes with version 0.7.8.

Without this change, the eth0 interface will never get an IP address on
Wheezy.

Change-Id: If40ed3b82ca78280190b66c06a0a77bb71270d7d
Merge "debian: fix network on Wheezy"