basic: include only what we use

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

src/basic/escape.h

@@ -22,8 +22,12 @@
 ***/
 
 #include <inttypes.h>
+#include <stddef.h>
+#include <stdint.h>
 #include <sys/types.h>
 
+#include "string-util.h"
+
 /* What characters are special in the shell? */
 /* must be escaped outside and inside double-quotes */
 #define SHELL_NEED_ESCAPE "\"\\`$"