gn: Downgrade GN to OpenHarmony compatible revision

OpenHarmony build system relies on the obsolete set_sources_assignment_filter
GN function which was recently dropped.

Signed-off-by: Robert Drab <robert.drab@huawei.com>
Signed-off-by: Esben Haabendal <esben.haabendal@huawei.com>

recipes-devtools/gn/gn_%.bbappend

+# SPDX-FileCopyrightText: Huawei Inc.
+#
+# SPDX-License-Identifier: Apache-2.0
+
+# OpenHarmony build system relies on the obsolete set_sources_assignment_filter
+# GN function which was recently dropped, therefore downgrading GN to revision
+# compatible with OpenHarmony
+
+SRCREV:oniro-openharmony-linux = "5da62d5e9d0f10cb8ece7c30563a6a214c78b68d"