bindings: rust: skip building all rust crates by default

bindings: rust: skip building all rust crates by default

We don't really need to build all the available rust crates here by
default, but only what's required by the libgpiod crate.

Currently we try to build gpiosim-sys crate as well, even if
"--enable-tests" isn't passed to autogen.sh, which results in following
build failure:

  error: could not find native static library `gpiosim`, perhaps an -L flag is missing?

Fix this by building just the libgpiod crate instead, which can force
building of the other crates based on the enabled configuration options.

Reported-by: Bartosz Golaszewski <bartosz.golaszewski@linaro.org>
Signed-off-by: Viresh Kumar <viresh.kumar@linaro.org>
Signed-off-by: Bartosz Golaszewski <bartosz.golaszewski@linaro.org>