build: automake complains IntegrationGuide is missing

build: automake complains IntegrationGuide is missing

Change set of 502c471406b32e5afcdea62fa8307f9856d05437 added IntegrationGuide,
but it wasn't added to EXTRA_DIST. So automake complains.
This patch adds the file to EXTRA_DIST.

> make[3]: Leaving directory `/openvswitch/build/datapath'
> The distribution is missing the following files:
> IntegrationGuide
> make[2]: *** [dist-hook-git] Error 1
> make[2]: *** Waiting for unfinished jobs....
> make[2]: Leaving directory `/openvswitch/build'
> make[1]: *** [all-recursive] Error 1
> make[1]: Leaving directory `/openvswitch/build'
> make: *** [all] Error 2

Signed-off-by: Isaku Yamahata <yamahata@valinux.co.jp>
Signed-off-by: Ben Pfaff <blp@nicira.com>