igvm: fix build when igvm is not installed in a well known path
`stubs/igvm.c` includes `igvm-internal.h`, that in turn includes the igvm library. This is required just for the QIgvm declaration and creates an unnecessary dependency on the library. Since igvm is not listed as a dependency of stubs, this results in build failing when the library is not installed in a "well known" path of the system. Add a forward declaration for QIgvm and move the definition of `qigvm_directive_madt` to `igvm.h` so that we can drop `igvm-internal.h` from `stubs/igvm.c`. Fixes: dea1f68a5c ("igvm: Fill MADT IGVM parameter field on x86_64") Suggested-by: Stefano Garzarella <sgarzare@redhat.com> Signed-off-by: Luigi Leonardi <leonardi@redhat.com> Message-ID: <20260309-master-v2-1-7e02f07a1096@redhat.com> Signed-off-by: Gerd Hoffmann <kraxel@redhat.com>