siphash.h: include stdint.

Doing this gives us a valid uint64_t type, freeing us from
dependencies on include order.

src/ext/siphash.h

@@ -1,6 +1,8 @@
 #ifndef SIPHASH_H
 #define SIPHASH_H
 
+#include <stdint.h>
+
 struct sipkey {
   uint64_t k0;
   uint64_t k1;