link libmetrics with libxml

libmetrics uses libxml, so link with it.

Index: vhostmd-1.1/libmetrics/Makefile.am
===================================================================
--- vhostmd-1.1.orig/libmetrics/Makefile.am
+++ vhostmd-1.1/libmetrics/Makefile.am
@@ -19,3 +19,5 @@ libmetrics_la_SOURCES =  \
 
 libmetrics_la_DEPENDENCIES = \
      libmetrics.h
+
+libmetrics_la_LIBADD = $(LIBXML_LIBS)