STM32 : correct compilation warnings

targets/TARGET_STM/stm_spi_api.c

@@ -67,6 +67,10 @@
 /* Consider 10ms as the default timeout for sending/receving 1 byte */
 #define TIMEOUT_1_BYTE 10
 
+#if defined(SPI_FLAG_FRLVL) // STM32F0 STM32F3 STM32F7 STM32L4
+extern HAL_StatusTypeDef HAL_SPIEx_FlushRxFifo(SPI_HandleTypeDef *hspi);
+#endif
+
 void init_spi(spi_t *obj)
 {
     struct spi_s *spiobj = SPI_S(obj);