linux-libc-headers: Install ashmem.h header file from drivers/staging

OpenHarmony requires Android Shared Memory kernel support which is currently
under drivers/staging/android/ and therefore the header file is not being
installed.

Signed-off-by: Robert Drab <robert.drab@huawei.com>

recipes-kernel/linux-libc-headers/linux-libc-headers_%.bbappend

+# SPDX-FileCopyrightText: Huawei Inc.
+#
+# SPDX-License-Identifier: Apache-2.0
+
+# OpenHarmony relies on Android's ashmem, which is in staging and therefore
+# the header is not installed by default
+do_install:append:oniro-openharmony-linux() {
+    install ${S}/drivers/staging/android/uapi/ashmem.h ${D}${includedir}/linux
+}