SPIFBlockDevice.h: include SingletonPtr.h

SPIFBlockDevice was using SingletonPtr without an include,
and only getting it via SPI.h.

Spotted while changing SPI to not use SingletonPtr - now
abandoned, but still this shouldn't have been relying on it.

Author: kjbracey

components/storage/blockdevice/COMPONENT_SPIF/SPIFBlockDevice.h

@@ -16,6 +16,7 @@
 #ifndef MBED_SPIF_BLOCK_DEVICE_H
 #define MBED_SPIF_BLOCK_DEVICE_H
 
+#include "platform/SingletonPtr.h"
 #include "SPI.h"
 #include "DigitalOut.h"
 #include "BlockDevice.h"