[platform] fix doxygen group close

sg- · sg- · commit dcbb6f6ab8de · 2017-04-26T09:14:36.000-05:00
diff --git a/platform/toolchain.h b/platform/toolchain.h
@@ -24,3 +24,5 @@
 #include "platform/mbed_toolchain.h"
 
 #endif
+
+/** @}*/
