basic: include only what we use

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

src/basic/siphash24.h

@@ -1,6 +1,8 @@
 #pragma once
 
 #include <inttypes.h>
+#include <stddef.h>
+#include <stdint.h>
 #include <sys/types.h>
 
 struct siphash {