Avoid possibly including an old vswitch-idl.h.

Avoid possibly including an old vswitch-idl.h.

Codes that uses #include "vswitch-idl.h" can get an older version of this
header, because this header file moved from vswitchd/ to lib/ and the
older generated file might still be present.

This helps out two ways:

     * "make clean" will delete the generated files from their old
       locations.

     * Use #include "lib/vswitch-idl.h" to explicitly avoid including the
       files from their old locations.

Reported-by: Justin Pettit <jpettit@nicira.com>
Signed-off-by: Ben Pfaff <blp@nicira.com>