www/geckodriver: Bump libc crate version again

$ make check-sanity
/!\ geckodriver-0.24.0: Makefile warnings, please consider fixing /!\

CARGO_CRATES=libc-0.2.43 may be unstable on aarch64 or not build on armv6,
armv7, powerpc64. Consider updating to the latest version (higher than 0.2.49).

This will give it at least a chance to build on tier 2 archs. We
bumped libc earlier but this was regressed on in r506813.

The libc warnings in USES=cargo do not seem to be enough. Make
them errors instead.

With hat: rust