net: socket: fix sys/_timespec.h not found error

When building `native_sim` with CPP & POSIX enabled, the
POSIX's `unistd.h` header causes a compilation error as
`sys/_timespec.h` doesn't exist in host gcc lib.

Remove the 'zephyr/posix' prefix, and rely on
`CMakeLists.txt`s to do the routing accordingly:
- zephyr/lib/CMakeLists.txt:L11
- zephyr/lib/posix/options/CMakeLists.txt:L8

Signed-off-by: Yong Cong Sin <ycsin@meta.com>

Author: ycsin

include/zephyr/net/socket.h

@@ -1403,21 +1403,11 @@ struct net_socket_register {
  * zephyr/net/socket.h header file.
  */
 #if defined(CONFIG_POSIX_API)
-#if !defined(ZEPHYR_INCLUDE_POSIX_ARPA_INET_H_)
-#include <zephyr/posix/arpa/inet.h>
-#endif
-#if !defined(ZEPHYR_INCLUDE_POSIX_NETDB_H_)
-#include <zephyr/posix/netdb.h>
-#endif
-#if !defined(ZEPHYR_INCLUDE_POSIX_UNISTD_H_)
-#include <zephyr/posix/unistd.h>
-#endif
-#if !defined(ZEPHYR_INCLUDE_POSIX_POLL_H_)
-#include <zephyr/posix/poll.h>
-#endif
-#if !defined(ZEPHYR_INCLUDE_POSIX_SYS_SOCKET_H_)
-#include <zephyr/posix/sys/socket.h>
-#endif
+#include <arpa/inet.h>
+#include <netdb.h>
+#include <unistd.h>
+#include <poll.h>
+#include <sys/socket.h>
 #endif /* CONFIG_POSIX_API */
 
 #endif /* ZEPHYR_INCLUDE_NET_SOCKET_H_ */