Add stdarg.c as va_list requires

https://github.com/Hamlib/Hamlib/issues/1644

include/hamlib/rig.h

@@ -37,6 +37,7 @@
 #define __FILENAME__ (strrchr(__FILE__, '/') ? strrchr(__FILE__, '/') + 1 : __FILE__)
 
 #include <stdio.h>
+#include <stdarg.h>
 #include <string.h>
 #include <inttypes.h>
 #include <time.h>