libxl: Fix missing dependency in api check rule

libxl: Fix missing dependency in api check rule

Without this, the api check cpp run might happen before the various
autogenerated files which are #include by libxl.h are ready.

We need to remove the api-ok file from AUTOINCS to avoid a circular
dependency.  Instead, we list it explicitly as a dependency of the
object files.  The result is that the api check is the last thing to
be done before make considers the preparation done and can start work
on compiling .c files into .o's.

Reported-by: Olaf Hering <olaf@aepfle.de>
Signed-off-by: Ian Jackson <ian.jackson@eu.citrix.com>
Tested-by: Olaf Hering <olaf@aepfle.de>
Acked-by: Ian Campbell <ian.campbell@citrix.com>
Committed-by: Ian Campbell <ian.campbell@citrix.com>