tests should ignore setservent() and endservent() on illumos

Depending on the compilation environment (e.g., whether the 3XNET or
3SOCKET version of these functions is in play, whether EXTENSIONS has
been defined, etc) these functions may be declared to return either void
or int.  The return value is hard coded as zero, and can be ignored to
better align with other platforms where these functions are always void.

libc-test/build.rs

@@ -866,6 +866,11 @@ fn test_solarish(target: &str) {
             "madvise" | "mprotect" if is_illumos => true,
             "door_call" | "door_return" | "door_create" if is_illumos => true,
 
+            // These functions may return int or void depending on the exact
+            // configuration of the compilation environment, but the return
+            // value is not useful (always 0) so we can ignore it:
+            "setservent" | "endservent" if is_illumos => true,
+
             _ => false,
         }
     });