ohos: Add shm_open and shm_unlink

OpenHarmony 4.1 adds support for `shm_open` and
`shm_unlink` so we can expose them unconditionally.
Users developing for older OpenHarmony versions
will only encounter a linker error if they attempt to use
the functions.
See OpenHarmony release notes for 4.1:
https://gitee.com/openharmony/docs/blob/master/en/release-notes/OpenHarmony-v4.1-release.md#arkcompiler

Signed-off-by: Jonathan Schwender <jonathan.schwender@huawei.com>

Author: jschwe

src/unix/linux_like/linux/mod.rs

@@ -5392,9 +5392,6 @@ cfg_if! {
                 spbufp: *mut *mut spwd,
             ) -> ::c_int;
 
-            pub fn shm_open(name: *const c_char, oflag: ::c_int, mode: mode_t) -> ::c_int;
-            pub fn shm_unlink(name: *const ::c_char) -> ::c_int;
-
             pub fn mq_open(name: *const ::c_char, oflag: ::c_int, ...) -> ::mqd_t;
             pub fn mq_close(mqd: ::mqd_t) -> ::c_int;
             pub fn mq_unlink(name: *const ::c_char) -> ::c_int;
@@ -5481,6 +5478,9 @@ extern "C" {
 
     pub fn getspnam(name: *const ::c_char) -> *mut spwd;
 
+    pub fn shm_open(name: *const c_char, oflag: ::c_int, mode: mode_t) -> ::c_int;
+    pub fn shm_unlink(name: *const ::c_char) -> ::c_int;
+
     // System V IPC
     pub fn shmget(key: ::key_t, size: ::size_t, shmflg: ::c_int) -> ::c_int;
     pub fn shmat(shmid: ::c_int, shmaddr: *const ::c_void, shmflg: ::c_int) -> *mut ::c_void;