Fix build without xen

Commit 11567cf introduced an include which will only work when
building with xen (particularly libxl).  However, that file is supposed
to be includable from anywhere (as with other testutils* files.

Signed-off-by: Martin Kletzander <mkletzan@redhat.com>

tests/testutilsxen.h

@@ -2,7 +2,9 @@
 # define _TESTUTILSXEN_H_
 
 # include "capabilities.h"
-# include "libxl/libxl_capabilities.h"
+# ifdef WITH_LIBXL
+#  include "libxl/libxl_capabilities.h"
+# endif
 
 virCapsPtr testXenCapsInit(void);