ci: Fix VPATH builds

The CI build machinery is intentionally not handled by autotools,
so for VPATH builds - which are mandatory now - we need to make
sure we're looking into $(srcdir).
Signed-off-by: NAndrea Bolognani <abologna@redhat.com>
Reviewed-by: NDaniel P. Berrangé <berrange@redhat.com>

Makefile.am

@@ -148,4 +148,4 @@ gen-AUTHORS:
 	fi
 
 ci-%:
-	$(MAKE) -C ci/ $@
+	$(MAKE) -C $(srcdir)/ci/ $@