libxl: Get compiler to warn about gc_opt==NULL

libxl: Get compiler to warn about gc_opt==NULL

Since it used to be legal to pass gc_opt==NULL, and there are various
patches floating about and under development which do so, add a
compiler annotation which makes the build fail when that is done.

This turns a runtime crash into a build failure, and should ensure
that we don't accidentally commit a broken combination of patches.

This is something of an annoying approach because it adds a macro
invocation to the RHS of every declaration of a function taking a
gc_opt.  So it should be reverted after Xen 4.2rc1.

Signed-off-by: Ian Jackson <ian.jackson@eu.citrix.com>
Acked-by: Ian Campbell <ian.campbell@citrix.com>
Committed-by: Ian Jackson <Ian.Jackson@eu.citrix.com>