basic + fundamental: Clean up includes

Split out of #37344.

src/basic/strxcpyx.c

@@ -11,11 +11,9 @@
  * occur outside of a loop where this is the preferred behavior.
  */
 
-#include <stdarg.h>
 #include <stdio.h>
 #include <string.h>
 
-#include "string-util.h"
 #include "strxcpyx.h"
 
 size_t strnpcpy_full(char **dest, size_t size, const char *src, size_t len, bool *ret_truncated) {