include: don't mention stub headers more than once in a make rule

When !GRANT_TABLE and !PV_SHIM headers-n contains grant_table.h twice,
causing make to complain "target '...' given more than once in the same
rule" for the rule generating the stub headers. We don't need duplicate
entries in headers-n anywhere, so zap them (by using $(sort ...)) right
where the final value of the variable is constructed.

Fixes: 6bec713f87 ("include/compat: produce stubs for headers not otherwise generated")
Signed-off-by: Jan Beulich <jbeulich@suse.com>
Reviewed-by: Anthony PERARD <anthony.perard@citrix.com>

xen/include/Makefile

@@ -34,7 +34,7 @@ headers-$(CONFIG_TRACEBUFFER) += compat/trace.h
 headers-$(CONFIG_XENOPROF) += compat/xenoprof.h
 headers-$(CONFIG_XSM_FLASK) += compat/xsm/flask_op.h
 
-headers-n := $(filter-out $(headers-y),$(headers-n) $(headers-))
+headers-n := $(sort $(filter-out $(headers-y),$(headers-n) $(headers-)))
 
 cppflags-y                := -include public/xen-compat.h -DXEN_GENERATING_COMPAT_HEADERS
 cppflags-$(CONFIG_X86)    += -m32