zephyr: remove posix prefix on standard headers

Instead of using `#include <posix/unistd.h>`,
just use `#include <unistd.h>`, etc.

Required-by zephyrproject-rtos/zephyr#43998

Signed-off-by: Christopher Friedt <chrisfriedt@gmail.com>

src/civetweb.c

@@ -163,21 +163,17 @@ mg_static_assert(sizeof(void *) >= sizeof(int), "data type size check");
 #endif /* __SYMBIAN32__ */
 
 #if defined(__ZEPHYR__)
-#include <time.h>
-
 #include <ctype.h>
-#include <net/socket.h>
-#include <posix/pthread.h>
-#include <posix/time.h>
+#include <sys/socket.h>
+#include <pthread.h>
+#include <time.h>
 #include <stdio.h>
 #include <stdlib.h>
 #include <string.h>
 #include <zephyr.h>
 
 #include <fcntl.h>
 
-#include <libc_extensions.h>
-
 /* Max worker threads is the max of pthreads minus the main application thread
  * and minus the main civetweb thread, thus -2
  */