vppapigen: support per-file (major,minor,patch) version stamps

Add one of these statements to foo.api:

  vl_api_version 1.2.3

to generate a version tuple stanza in foo.api.h:

/****** Version tuple *****/

vl_api_version_tuple(foo, 1, 2, 3)

Change-Id: Ic514439e4677999daa8463a94f948f76b132ff15
Signed-off-by: Dave Barach <dave@barachs.net>
Signed-off-by: Ole Troan <ot@cisco.com>
diff --git a/src/vnet/session/session.api b/src/vnet/session/session.api
index 30d2ae9..992a063 100644
--- a/src/vnet/session/session.api
+++ b/src/vnet/session/session.api
@@ -12,7 +12,9 @@
  * See the License for the specific language governing permissions and
  * limitations under the License.
  */
- 
+
+vl_api_version 1.0.0
+
 /** \brief client->vpp, attach application to session layer
     @param client_index - opaque cookie to identify the sender
     @param context - sender context, to match reply w/ request