[restructure] Fixed small include error due to indirect dependency

previous dependencies
FileBase.h -> PlatformMutex.h -> mbed_platform.h
SingletonPtr.h -> PlatformMutex.h -> mbed_platform.h -> stddef.h

now
FileBase.h -> mbed_platform.h
SingletonPtr.h -> stddef.h

Author: geky

drivers/FileBase.h

@@ -40,9 +40,7 @@ typedef long off_t;
 #    include <sys/syslimits.h>
 #endif
 
-#include "PlatformMutex.h"
-#include "SingletonPtr.h"
-#include "PlatformMutex.h"
+#include "mbed_platform.h"
 
 namespace mbed {
 

platform/SingletonPtr.h

@@ -17,6 +17,7 @@
 #define SINGLETONPTR_H
 
 #include <stdint.h>
+#include <stddef.h>
 #include <new>
 #include "mbed_assert.h"
 #ifdef MBED_CONF_RTOS_PRESENT