[gbinder] Set function attributes. JB#42956

For better compile time diagnostics.

include/gbinder_servicemanager.h

 /*
- * Copyright (C) 2018-2019 Jolla Ltd.
- * Copyright (C) 2018-2019 Slava Monich <slava.monich@jolla.com>
+ * Copyright (C) 2018-2020 Jolla Ltd.
+ * Copyright (C) 2018-2020 Slava Monich <slava.monich@jolla.com>
  *
  * You may use this file under the terms of BSD license as follows:
  *
@@ -14,8 +14,8 @@
  *      notice, this list of conditions and the following disclaimer in the
  *      documentation and/or other materials provided with the distribution.
  *   3. Neither the names of the copyright holders nor the names of its
- *      contributors may be used to endorse or promote products derived from
- *      this software without specific prior written permission.
+ *      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
@@ -93,14 +93,16 @@ gbinder_servicemanager_new_local_object(
     GBinderServiceManager* sm,
     const char* iface,
     GBinderLocalTransactFunc handler,
-    void* user_data);
+    void* user_data)
+    G_GNUC_WARN_UNUSED_RESULT;
 
 GBinderLocalObject*
 gbinder_servicemanager_new_local_object2(
     GBinderServiceManager* sm,
     const char* const* ifaces,
     GBinderLocalTransactFunc handler,
-    void* user_data); /* Since 1.0.29 */
+    void* user_data) /* Since 1.0.29 */
+    G_GNUC_WARN_UNUSED_RESULT;
 
 GBinderServiceManager*
 gbinder_servicemanager_ref(
@@ -127,7 +129,9 @@ gbinder_servicemanager_list(
 
 char**
 gbinder_servicemanager_list_sync(
-    GBinderServiceManager* sm);
+    GBinderServiceManager* sm)
+    G_GNUC_WARN_UNUSED_RESULT
+    G_GNUC_MALLOC;
 
 gulong
 gbinder_servicemanager_get_service(