AP_HAL_AVR: fixup in vprintf header

libraries/AP_HAL_AVR/utility/vprintf.h

@@ -3,6 +3,7 @@
 #define __AP_HAL_AVR_UTILITY_VPRINTF_H__
 
 #include <AP_HAL.h>
+#include <stdarg.h>
 
 void vprintf (AP_HAL::Print *s, unsigned char in_progmem, const char *fmt, va_list ap);