Bump VRT to 6.1

It should have been done for 5.1.2 but hasn't. This is a minor bump of a
single addition so the impact is limited. I also left the release date
to "unreleased" on purpose, since it technically hasn't been released as
6.1 yet.

include/vrt.h

@@ -67,7 +67,7 @@
 
 #define VRT_MAJOR_VERSION	6U
 
-#define VRT_MINOR_VERSION	0U
+#define VRT_MINOR_VERSION	1U
 
 
 /***********************************************************************/