disas/libvixl: Really suppress gcc 4.6.3 sign-compare warnings

Commit 8acc216b attempted to silence some sign-compare
warnings in libvixl by adding -Wno-sign-compare to the CFLAGS
for the relevant objects. Unfortunately it was ineffective
because it was placed before $(QEMU_CFLAGS), so the -Wall in
the general flags overrode -Wno-sign-compare rather than
vice-versa. Reorder the flags so the warning suppression works.

Thanks to Franz-Josef Haider <Franz-Josef.Haider@student.uibk.ac.at>
for pointing out what was wrong with the original patch.

Signed-off-by: Peter Maydell <peter.maydell@linaro.org>
Reviewed-by: Alex Bennée <alex.bennee@linaro.org>
Tested-by: Alex Bennée <alex.bennee@linaro.org>
Message-id: 1452783202-576-1-git-send-email-peter.maydell@linaro.org