qapi-schema.json has been split into three smaller JSON files in qapi/.
Add them as dependencies for the code generation in the Makefile, so
changes to them will result in a rebuilt of all QAPI-dependent code.

Signed-off-by: Max Reitz <mreitz@redhat.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>
Reviewed-by: Eric Blake <eblake@redhat.com>
Signed-off-by: Luiz Capitulino <lcapitulino@redhat.com>