enable utmpx feature for musl

musl provides stubs of those functions, and those stubs are available in the
libc crate version 0.2.172. Thus let's enable the feature so that it can
compile with musl.  Note that those stubs always return a success exit value,
and commands such as "users" will report an empty list of users, when calling
those stubs.

This is consistent with the behavior of GNU coreutils which does the same thing.

The coreutils utillities using utmpx are "pinky", "uptime", "users", "who".
This is the expected behavior when using those utilities compiled with those musl utmpx stubs:
```
root@qemuarm64:~# users
root@qemuarm64:~# echo $?
0
root@qemuarm64:~# pinky
Login    Name                 TTY      Idle   When         Where
root@qemuarm64:~# echo $?
0
root@qemuarm64:~# uptime
 12:58:47 up 0 min,  0 users,  load average: 0.07, 0.02, 0.00
root@qemuarm64:~# echo $?
0
root@qemuarm64:~# who
root@qemuarm64:~# echo $?
0
```

Closes #1361

Signed-off-by: Etienne Cordonnier <ecordonnier@snap.com>

Author: Ecordonnier

Cargo.toml

@@ -178,6 +178,7 @@ feat_os_unix_musl = [
   "feat_require_crate_cpp",
   "feat_require_unix",
   "feat_require_unix_hostid",
+  "feat_require_unix_utmpx",
 ]
 feat_os_unix_android = [
   "feat_Tier1",

src/uucore/src/lib/features.rs

@@ -76,7 +76,6 @@ pub mod signals;
     not(target_os = "fuchsia"),
     not(target_os = "openbsd"),
     not(target_os = "redox"),
-    not(target_env = "musl"),
     feature = "utmpx"
 ))]
 pub mod utmpx;

src/uucore/src/lib/lib.rs

@@ -89,7 +89,6 @@ pub use crate::features::signals;
     not(target_os = "fuchsia"),
     not(target_os = "openbsd"),
     not(target_os = "redox"),
-    not(target_env = "musl"),
     feature = "utmpx"
 ))]
 pub use crate::features::utmpx;