tweaks: move an include to the file that actually makes use of it

Well, it will compile even without that include.  :|
I don't know why, since it does use va_list.

src/files.c

@@ -21,6 +21,7 @@
 
 #include "proto.h"
 
+#include <stdarg.h>
 #include <stdio.h>
 #include <string.h>
 #include <unistd.h>