Make clock source changeable over mbed_app.json

By adding the missing include the clock source for EFM32-Targets is changeable over mbed_app.json.

Author: steinerphilipp

targets/TARGET_Silicon_Labs/TARGET_EFM32/common/mbed_overrides.c

@@ -29,6 +29,7 @@
 #include "device.h"
 #include "em_usart.h"
 #include "gpio_api.h"
+#include "clocking.h"
 
 gpio_t bc_enable;