[libxml2] Fix macro redefinition warning

[Nick Wellnhofer <nwellnhof src gnome org>]

commit 8bc6c878e9e3cc2941eb1264693841ece84ceaaa
Author: J. Peter Mugaas <jpmugaas suddenlink net>
Date:   Sat Oct 21 13:57:44 2017 +0200

    Fix macro redefinition warning

 libxml.h |    2 +-
 1 files changed, 1 insertions(+), 1 deletions(-)
---
diff --git a/libxml.h b/libxml.h
index f13e9de..4fe56d2 100644
--- a/libxml.h
+++ b/libxml.h
@@ -109,7 +109,7 @@ int xmlNop(void);
 #endif
 #endif
 #endif
-#if !defined(PIC) && !defined(NOLIBTOOL)
+#if !defined(PIC) && !defined(NOLIBTOOL) && !defined(LIBXML_STATIC)
 #  define LIBXML_STATIC
 #endif
 #endif /* ! __XML_LIBXML_H__ */