Patch libxml used ofr docbook to work with current zlib

Patch from https://git.gnome.org/browse/libxml2/commit/?id=a7e79f28689c574e0bbef17f4cb3da00249181ff

* Fixes #6958
* We should probably use the host libxml2 instead.

src/tools/docbook/libxml2/xmlIO.c

@@ -2298,6 +2298,9 @@ __xmlParserInputBufferCreateFilename(const char *URI, xmlCharEncoding enc) {
 #ifdef HAVE_ZLIB_H
 	if ((xmlInputCallbackTable[i].opencallback == xmlGzfileOpen) &&
 		(strcmp(URI, "-") != 0)) {
+#if defined(ZLIB_VERNUM) && ZLIB_VERNUM >= 0x1230
+		ret->compressed = !gzdirect(context);
+#else
 	    if (((z_stream *)context)->avail_in > 4) {
 	        char *cptr, buff4[4];
 		cptr = (char *) ((z_stream *)context)->next_in;
@@ -2309,6 +2312,7 @@ __xmlParserInputBufferCreateFilename(const char *URI, xmlCharEncoding enc) {
 		    gzrewind(context);
 		}
 	    }
+#endif
 	}
 #endif
     }