Fix build error on Android

[taiki-e]

Currently building for Android fails with the following errors:

$ cargo build --target=aarch64-linux-android --features=std
   Compiling getrandom v0.3.0 (/Users/taiki/projects/forks/others/getrandom)
   Compiling cfg-if v1.0.0
error[E0432]: unresolved import `libc`
 --> src/backends/../util_libc.rs:6:13
  |
6 |         use libc::__errno as errno_location;
  |             ^^^^ use of unresolved module or unlinked crate `libc`
  |
  = help: if you wanted to use a crate named `libc`, use `cargo add libc` to add it to your `Cargo.toml`

error[E0433]: failed to resolve: use of unresolved module or unlinked crate `libc`
  --> src/backends/../util_libc.rs:68:47
   |
68 |                 if err.raw_os_error() != Some(libc::EINTR) {
   |                                               ^^^^ use of unresolved module or unlinked crate `libc`
   |
   = help: if you wanted to use a crate named `libc`, use `cargo add libc` to add it to your `Cargo.toml`

<ommited>

error[E0433]: failed to resolve: use of unresolved module or unlinked crate `libc`
   --> src/backends/use_file.rs:207:23
    |
207 |         let mut pfd = libc::pollfd {
    |                       ^^^^ use of unresolved module or unlinked crate `libc`
    |
    = help: if you wanted to use a crate named `libc`, use `cargo add libc` to add it to your `Cargo.toml`

This is because android set target_env to "", but the following condition has not(any(target_env = "", ..)) for both Linux and Android:

getrandom/Cargo.toml

Lines 33 to 35 in aa96363

	# linux_android / linux_android_with_fallback
	[target.'cfg(all(any(target_os = "linux", target_os = "android"), not(any(target_env = "", getrandom_backend = "custom"))))'.dependencies]
	libc = { version = "0.2.154", default-features = false }

[taiki-e]

As for CI failure in "Build / Apple Other", it will probably be resolved in the next nightly: rust-lang/rust#136098