vapi: Missing include file in vlib.api.vapi.h

vapi: Missing include file in vlib.api.vapi.h

- A call to vapi_msg_control_ping_hton() is generated in
  a static inline function in vlib.api.vapi.h, which is
  defined/generated in memclnt.api.vapi.h without
  including memclnt.api.vapi.h in vlib.api.vapi.h.
  This breaks the compilation of plugins which include
  only vlib.api.vapi.h (e.g. hicn_plugin from the HICN
  project).

Type: fix
Fixes: a1400cecb

Signed-off-by: Dave Wallace <dwallacelf@gmail.com>
Change-Id: I5574f4ed496183ea93265f493bf3624254a865a2
(cherry picked from commit 14baada2140ad5a19e4d502e81296fe70b5b54d1)