Add missing <stdarg.h> include for va_list
  
  Fixes the build on uClibc.
  
  Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>
  
  Index: b/src/common/log.h
  ===================================================================
  --- a/src/common/log.h
  +++ b/src/common/log.h
  @@ -13,6 +13,7 @@
   
   #include <stdio.h>
   #include <stdlib.h>
  +#include <stdarg.h>
   #include "common.h"
   
   class Log