Ensure xmlSaveToBuffer is always defined

RHEL4 vintage libxml2 header files are missing xmlSaveToBuffer
despite the symbol existing in the binary

Signed-off-by: Daniel P. Berrange <berrange@redhat.com>

tools/virsh-domain.c

@@ -56,6 +56,17 @@
 #include "virtypedparam.h"
 #include "virxml.h"
 
+/* libxml2 in RHEL4 has this symbol in the binary but it
+ * is commented out in the header, despite apparently
+ * working fine. This hacks around that header problem
+ */
+#ifndef xmlSaveToBuffer
+XMLPUBFUN xmlSaveCtxtPtr XMLCALL
+xmlSaveToBuffer         (xmlBufferPtr buffer,
+                         const char *encoding,
+                         int options);
+#endif
+
 /* Gnulib doesn't guarantee SA_SIGINFO support.  */
 #ifndef SA_SIGINFO
 # define SA_SIGINFO 0