xen/Makefile: Correct XEN_EXTERVERSION for unstable

Signed-off-by: Julien Grall <[email protected]>

xen/Makefile

@@ -6,7 +6,7 @@ this-makefile := $(call lastword,$(MAKEFILE_LIST))
 # All other places this is stored (eg. compile.h) should be autogenerated.
 export XEN_VERSION       = 4
 export XEN_SUBVERSION    = 20
-export XEN_EXTRAVERSION ?= $(XEN_VENDORVERSION)
+export XEN_EXTRAVERSION ?= -unstable$(XEN_VENDORVERSION)
 export XEN_FULLVERSION   = $(XEN_VERSION).$(XEN_SUBVERSION)$(XEN_EXTRAVERSION)
 -include xen-version