[gn] include lib/platform files in build (#6317)

These are needed to enable logging when building with gn.
This fixes the bug #6316.

src/lib/platform/BUILD.gn

@@ -0,0 +1,42 @@
+#  Copyright (c) 2021, The OpenThread Authors.
+#  All rights reserved.
+#
+#  Redistribution and use in source and binary forms, with or without
+#  modification, are permitted provided that the following conditions are met:
+#  1. Redistributions of source code must retain the above copyright
+#     notice, this list of conditions and the following disclaimer.
+#  2. Redistributions in binary form must reproduce the above copyright
+#     notice, this list of conditions and the following disclaimer in the
+#     documentation and/or other materials provided with the distribution.
+#  3. Neither the name of the copyright holder nor the
+#     names of its contributors may be used to endorse or promote products
+#     derived from this software without specific prior written permission.
+#
+#  THIS SOFTWARE IS PROVIDED BY THE COPYRIGHT HOLDERS AND CONTRIBUTORS "AS IS"
+#  AND ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT LIMITED TO, THE
+#  IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR PURPOSE
+#  ARE DISCLAIMED. IN NO EVENT SHALL THE COPYRIGHT HOLDER OR CONTRIBUTORS BE
+#  LIABLE FOR ANY DIRECT, INDIRECT, INCIDENTAL, SPECIAL, EXEMPLARY, OR
+#  CONSEQUENTIAL DAMAGES (INCLUDING, BUT NOT LIMITED TO, PROCUREMENT OF
+#  SUBSTITUTE GOODS OR SERVICES; LOSS OF USE, DATA, OR PROFITS; OR BUSINESS
+#  INTERRUPTION) HOWEVER CAUSED AND ON ANY THEORY OF LIABILITY, WHETHER IN
+#  CONTRACT, STRICT LIABILITY, OR TORT (INCLUDING NEGLIGENCE OR OTHERWISE)
+#  ARISING IN ANY WAY OUT OF THE USE OF THIS SOFTWARE, EVEN IF ADVISED OF THE
+#  POSSIBILITY OF SUCH DAMAGE.
+#
+
+visibility = [ "../../../*" ]
+
+platform_sources = [
+  "exit_code.c",
+  "exit_code.h",
+]
+
+config("platform_config") {
+  include_dirs = [ ".." ]
+}
+
+static_library("libopenthread-platform") {
+  sources = platform_sources
+  public_configs = [ ":platform_config" ]
+}

src/lib/spinel/BUILD.gn

@@ -69,6 +69,7 @@ static_library("libopenthread-spinel-ncp") {
   public_deps = [
     ":spinel-api",
     "../../core:libopenthread_core_headers",
+    "../platform:libopenthread-platform",
   ]
   public_configs = [ ":spinel_config_openthread_message_enable" ]
 }
@@ -78,6 +79,7 @@ static_library("libopenthread-spinel-rcp") {
   public_deps = [
     ":spinel-api",
     "../../core:libopenthread_core_headers",
+    "../platform:libopenthread-platform",
   ]
   public_configs = [ ":spinel_config_openthread_message_disable" ]
 }