Add xz to xml2-config --libs output

XML_LIBS should include LZMA_LIBS. This ensures that 'xml2-config --libs'
shows -llzma when xz is enabled. Otherwise static link fails because of
missing xz symbols.
diff --git a/configure.ac b/configure.ac
index fd05c56..352da48 100644
--- a/configure.ac
+++ b/configure.ac
@@ -1526,7 +1526,7 @@
                 LDFLAGS="${_ldflags}"])]))
     fi
 fi
-XML_LIBS="-lxml2 $Z_LIBS $THREAD_LIBS $ICONV_LIBS $ICU_LIBS $M_LIBS $LIBS"
+XML_LIBS="-lxml2 $Z_LIBS $LZMA_LIBS $THREAD_LIBS $ICONV_LIBS $ICU_LIBS $M_LIBS $LIBS"
 XML_LIBTOOLLIBS="libxml2.la"
 AC_SUBST(WITH_ICU)