Add missing headers to detect-readline.c

Addresss issue #21.

build/detect-readline.c

@@ -1,4 +1,8 @@
/* 'info readline' shows an example with these three headers */

#include <stdio.h>  /* readline needs this, issue #21 */
#include <readline/readline.h>
#include <readline/history.h>

int main(void) {
  char *line = readline("");