Build the library via the main meson.build just like for capstone.
This improves the current state of affairs in that we will re-link
the qemu executables against a changed libfdt.a, which we wouldn't
do before-hand, and lets us remove the whole recursive make machinery.

Tested-by: Richard Henderson <richard.henderson@linaro.org>
Reviewed-by: Richard Henderson <richard.henderson@linaro.org>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>