Makefile: avoid non-literal printf format string warnings

The clang compiler by default dislikes non-literal format strings
in *printf functions, so it complains about kvm__set_dir() in kvm.c
and about the error reporting functions.
Since a fix is not easy and the code itself is fine (just seems that
the compiler is not smart enough to see that), let's just disable
the warning. Since GCC knows about this option as well (it just
doesn't have it enabled with -Wall), we can unconditionally add this
to the warning options.

Signed-off-by: Andre Przywara <andre.przywara@arm.com>
Signed-off-by: Will Deacon <will.deacon@arm.com>