Reenable xz support by default

For https://bugzilla.gnome.org/show_bug.cgi?id=757466

problem was introduced by commit f3f86ff465c92c79f834d7b981f3c7274a8bb5c8
for https://bugzilla.gnome.org/show_bug.cgi?id=711026
diff --git a/xzlib.c b/xzlib.c
index 1fab546..782957f 100644
--- a/xzlib.c
+++ b/xzlib.c
@@ -8,7 +8,7 @@
  */
 #define IN_LIBXML
 #include "libxml.h"
-#ifdef HAVE_LZMA_H
+#ifdef LIBXML_LZMA_ENABLED
 
 #include <string.h>
 #ifdef HAVE_ERRNO_H
@@ -34,7 +34,9 @@
 #ifdef HAVE_ZLIB_H
 #include <zlib.h>
 #endif
+#ifdef HAVE_LZMA_H
 #include <lzma.h>
+#endif
 
 #include "xzlib.h"
 #include <libxml/xmlmemory.h>
@@ -799,4 +801,4 @@
     xmlFree(state);
     return ret ? ret : LZMA_OK;
 }
-#endif /* HAVE_LZMA_H */
+#endif /* LIBXML_LZMA_ENABLED */