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

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: 6bec713f871f ("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>
master commit: 231ab79704cbb5b9be7700287c3b185225d34f1b
master date: 2023-03-28 14:20:16 +0200