emscripten: Remove sys/sysctl.h usage

See: emscripten-core/emscripten@15f7632.

(backport <#3962>)
(cherry picked from commit 6670ab2)

Author: kleisauke

libc-test/build.rs

@@ -2856,7 +2856,6 @@ fn test_emscripten(target: &str) {
                "sys/statvfs.h",
                "sys/swap.h",
                "sys/syscall.h",
-               "sys/sysctl.h",
                "sys/sysinfo.h",
                "sys/time.h",
                "sys/timerfd.h",