basic: include only what we use

This is a cleaned up result of running iwyu but without forward
declarations on src/basic.

src/basic/strxcpyx.c

@@ -25,6 +25,7 @@
  * Returns the * remaining size, and 0 if the string was truncated.
  */
 
+#include <stdarg.h>
 #include <stdio.h>
 #include <string.h>