Fix undefined reference in python module

For https://bugzilla.gnome.org/show_bug.cgi?id=622023
when compiled with LDFLAGS="${LDFLAGS} -Wl,-z,-defs -Wl,--no-undefined"
the python module would failed due to the undefined. This add an
explicit reference to python lib.
diff --git a/python/Makefile.am b/python/Makefile.am
index d2a2253..68d2236 100644
--- a/python/Makefile.am
+++ b/python/Makefile.am
@@ -31,7 +31,7 @@
 
 libxml2mod_la_SOURCES = $(srcdir)/libxml.c $(srcdir)/types.c
 nodist_libxml2mod_la_SOURCES = libxml2-py.c
-libxml2mod_la_LIBADD = $(mylibs) @CYGWIN_EXTRA_PYTHON_LIBADD@ @WIN32_EXTRA_PYTHON_LIBADD@ @PYTHON_LIBS@
+libxml2mod_la_LIBADD = $(mylibs) @CYGWIN_EXTRA_PYTHON_LIBADD@ @WIN32_EXTRA_PYTHON_LIBADD@ @PYTHON_LIBS@ -lpython$(PYTHON_VERSION)
 
 $(srcdir)/libxml.c: libxml2-py.h # to generate before to compile