tools: Workaround wrong use of tools/Rules.mk by qemu-trad

tools: Workaround wrong use of tools/Rules.mk by qemu-trad

qemu-trad build system, when built from xen.git, will make use of
Rules.mk (setup via qemu-trad.git/xen-setup). This mean that changes
to Rules.mk will have an impact our ability to build qemu-trad.

Recent commit e4f5949c4466 ("tools: Add -Werror by default to all
tools/") have added "-Werror" to the CFLAGS and qemu-trad start to use
it. But this fails and there's lots of warning that are now turned
into error.

We should teach qemu-trad and xen.git to not have to use Rules.mk when
building qemu-trad, but for now, avoid adding -Werror to CFLAGS when
building qemu-trad.

Signed-off-by: Anthony PERARD <anthony.perard@citrix.com>
Acked-by: Andrew Cooper <andrew.cooper3@citrix.com>
Release-acked-by: Henry Wang <Henry.Wang@arm.com>