Upgrade libc

[hack3ric]

wireguard-vanity-address failed to build on Arch Linux RISC-V due to outdated 5-year-old libc crate not recogizing newer architectures like RISC-V:

error[E0412]: cannot find type `c_char` in the crate root
  --> /build/.cargo/registry/src/index.crates.io-6f17d22bba15001f/libc-0.2.62/src/unix/mod.rs:43:29
   |
6  | pub type c_schar = i8;
   | ---------------------- similarly named type alias `c_schar` defined here
...
43 |         pub gr_name: *mut ::c_char,
   |                             ^^^^^^
   |
help: a type alias with a similar name exists
   |
43 |         pub gr_name: *mut ::c_schar,
   |                             ~~~~~~~
help: consider importing this type alias
   |
116+         use ffi::c_char;
   |
help: if you import `c_char`, refer to it directly
   |
43 -         pub gr_name: *mut ::c_char,
43 +         pub gr_name: *mut c_char,
   |
...

Full build log can be found at https://archriscv.felixc.at/.status/log.htm?url=logs/wireguard-vanity-address/wireguard-vanity-address-0.4.0-3.log.

This could be solved by simply calling cargo update -p libc, but the lockfile structure will be upgraded to newer version 3 format, which might break compatibility.