Export session.api.h for out-of-tree builds

session.api.h is included in src/vnet/vnet_all_api_h.h
since 68b0fb0c620, as such it should be added to the 'devel' package
and installed for out-of-tree builds which use binary VPP API

Change-Id: Ie8ad03d8df2a96cbc6913f767500d5ff34dcc915
Signed-off-by: Alexander Popovsky (apopovsk) <apopovsk@cisco.com>
diff --git a/src/vnet.am b/src/vnet.am
index 4e30ee9..ca24c9c 100644
--- a/src/vnet.am
+++ b/src/vnet.am
@@ -842,7 +842,8 @@
   vnet/session/session.h			\
   vnet/session/application.h			\
   vnet/session/transport.h			\
-  vnet/session/application_interface.h
+  vnet/session/application_interface.h		\
+  vnet/session/session.api.h
 
 API_FILES += vnet/session/session.api