rust: fix build when --disable-rust and meson < 1.9

Commit e65030ed50ee moved rust_std and build.rust_std from per-target
override_options into the project's default_options, in order to avoid
repetition.  However, default_options are validated unconditionally at
project initialization, even when Rust is disabled.  This breaks builds
with meson < 1.9.0 which does not know about "build.rust_std":

  meson.build:1:0: ERROR: Unknown option: "build.rust_std".

Make the options conditional on the meson version, since Rust only
supports new versions of Meson anyway.

Fixes: e65030ed50ee ("rust: remove unnecessary repetitive options")
Reported-by: Marc-André Lureau <marcandre.lureau@redhat.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>