Auto merge of #2428 - GrayJack:haiku-getgrouplist, r=JohnTitor

Haiku: Add getgrouplist function

Haiku actually does expose `getgrouplist` function to their public C API. You can see it in their [headers files](https://github.com/haiku/haiku/blob/8f16317a5b6db5c672f331814273e5857555020f/headers/posix/unistd.h#L275)

> P.S. I'm participating in [Hacktoberfest 2021](https://hacktoberfest.digitalocean.com/). If this PR is up to standard and merged, I'd appreciate if the `hacktoberfest-accepted` label could be added. Thanks!

Author: bors

src/unix/haiku/mod.rs

@@ -1510,6 +1510,12 @@ extern "C" {
         buflen: ::size_t,
         result: *mut *mut ::group,
     ) -> ::c_int;
+    pub fn getgrouplist(
+        user: *const ::c_char,
+        basegroup: ::gid_t,
+        grouplist: *mut ::gid_t,
+        groupcount: *mut ::c_int,
+    ) -> ::c_int;
     pub fn sigaltstack(ss: *const stack_t, oss: *mut stack_t) -> ::c_int;
     pub fn sem_close(sem: *mut sem_t) -> ::c_int;
     pub fn getdtablesize() -> ::c_int;