Don't forget to install xmlversion.h

Fixes 7f7961df.
diff --git a/include/libxml/Makefile.am b/include/libxml/Makefile.am
index cb717cf..5915e3e 100644
--- a/include/libxml/Makefile.am
+++ b/include/libxml/Makefile.am
@@ -49,4 +49,6 @@
 		xmlsave.h \
 		schematron.h
 
+nodist_xmlinc_HEADERS = xmlversion.h
+
 EXTRA_DIST = xmlversion.h.in