Don't include wasip2.h in public headers (#649)

Defines too many symbols for downstream consumers (fails Python's build)

Author: alexcrichton

libc-bottom-half/headers/public/__header_time.h

@@ -10,9 +10,7 @@
 #include <__struct_tm.h>
 #include <__typedef_clockid_t.h>
 
-#ifdef __wasilibc_use_wasip2
-#include <wasi/wasip2.h>
-#else
+#ifndef __wasilibc_use_wasip2
 #include <wasi/api.h>
 #endif