meson: pick libfdt from common_ss when building target-specific files

meson: pick libfdt from common_ss when building target-specific files

Avoid having to list dependencies such as libfdt twice, both on common_ss
and specific_ss.  Instead, just take all the dependencies in common_ss
and allow the target-specific libqemu-*.fa library to use them.

Reviewed-by: Philippe Mathieu-Daudé <philmd@linaro.org>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>